My campaign opens with PC's attending a royal wedding either as guests or servants for the celebratory fear. And naturally, weapons, equipment, etc are forbidden at the event. But of course, uninvited hostile guests show up. And I need a way for the PC's to have quick access to their stuff when the going gets tough. Are there any established small easily concealable magic items that could get them ready? Or will I have to think of something else?

My stock answer on this kind of situation is, that’s for the players to figure out. 
Your job is to set up the security, not to intentionally build a flaw in it, and therefore give the characters the 1 “correct” solution. 
Maybe they just go at the enemy unarmed. Maybe some of them have class features that let them summon their weapons, maybe they talk a guard into giving them their stuff after things go sideways, maybe they talk to the bad guys, maybe they knock out the guards and recover their things that way. There’s dozens of ways to solve the problem, don’t do it for them.

Small daggers and throwing spikes/ Stars are easy to hide a Garrote could be hidden as a wiry bangle  worn on your wrist, A kusarigama could be hidden in a waist sash...

Quote from Xalthu >>

My stock answer on this kind of situation is, that’s for the players to figure out. 
Your job is to set up the security, not to intentionally build a flaw in it, and therefore give the characters the 1 “correct” solution. 
Maybe they just go at the enemy unarmed. Maybe some of them have class features that let them summon their weapons, maybe they talk a guard into giving them their stuff after things go sideways, maybe they talk to the bad guys, maybe they knock out the guards and recover their things that way. There’s dozens of ways to solve the problem, don’t do it for them.

 Thanks.

Also, consider just having some of the fancy guards that get killed in the initial attack have some magic weapons that the characters can scavenge for the combat.

Broadly speaking, any well known method of doing that is also something that whoever is enforcing the 'no weapons' rule knows about and will forbid. So yes, problem for the PCs to solve. Just be aware that the PCs are likely to not be at 100% power and balance the encounter based on that assumption (even if the PCs can smuggle in weapons and spell foci, they probably won't be able to smuggle in their armor).

Quote from DruidofMN >>

My campaign opens with PC's attending a royal wedding either as guests or servants for the celebratory fear. And naturally, weapons, equipment, etc are forbidden at the event. But of course, uninvited hostile guests show up. And I need a way for the PC's to have quick access to their stuff when the going gets tough. Are there any established small easily concealable magic items that could get them ready? Or will I have to think of something else?

1) Why are weapons "naturally" forbidden? Casters and monks are more or less fully armed when naked anyway. Full armor or backpacks full of dungeoneering equipment wouldn't be appropriate, sure, but formal attire for a royal wedding could easily include a sword at your hip. This is doubly true if no trouble is expected, and everyone who's supposed to be there is an invited guest or (presumably) vetted servant. On the other hand, if there is a "no weapons" policy, there could be a plot reason for it -- if, for instance, this is some sort of political marriage between warring factions, and part of the reason the party is there is to make sure a fight doesn't break out before the I Do's... 

2) If you are sticking with weapons being verboten, consider having the captain of the guard, or whoever, get all the guests to deposit their weapons in one bag of holding or the like, so they would be easily accessible if there were some sort of trouble. Alternately, depending on the scenario, all the weapons could be in the castle armory, giving the party the choice of burning time they may not have to go retrieve them, or relying on whatever is at hand instead

3) Speaking of which, if your goal as the DM is to give the party the challenge of fending off the wedding crashers without use of their usual toys, then make sure there are alternatives available -- guards with non-magical weapons that can be "borrowed", ceremonial but still functional weapons on the walls, that sort of thing

One of the explicit uses of Sleight of Hand is concealing small weapons on your person.

There's a spell called Wristpocket. It's from ... something I've never heard of before, Explorers Guide to Wildemount? But anyways, it's there. You can hide one item. And it's a self cast, but still.

There are multiple ways to get past such security, Disguise Self spell, basically hide the weapons, invisibility spell, if no magic protection, just straight up walk past it. Some small weapons like Daggers might be able to be hidden anyway. A Druid could wildshape into a squirrel or a cat and carry equipment in, in their wildshape.They could even try to bride/intimidate/persuade the security to let them past with weapons... those skills exist for reasons.

Overall, you just need to drop a hint or two that having their weapons at the wedding would probably be a good idea. Of course this all relies on how tight security really is, if they are patting everybody down then something like Disguise Self might fail.

Quote from R3sistance >>

Overall, you just need to drop a hint or two that having their weapons at the wedding would probably be a good idea. Of course this all relies on how tight security really is, if they are patting everybody down then something like Disguise Self might fail.

 If there's meaningful security they'll have detect magic, which means you need to cast nystul's magic aura on any magic items you're using, and spells will just fail.

Which is, incidentally, hilariously broken in 5.24 (a vampire spellcaster can cast nystul's magic aura on itself to change creature type to humanoid -- and be immune to all spells and effects that normally target undead, such as protection from evil, turn undead, dispel evil and good, forbiddance, etc).

The trick here is to be clear on communication and make sure that expectations align.

While one player may be given this situation and think, "Hmm, my character doesn't feel safe going in here with no weapons. I need to think of ways to sneak them in or recover them in case things go sideways,"

another player might think, "The DM is taking away our weapons. I guess they want us to focus on roleplaying this session." Then when the conflict erupts, they feel deceived or tricked - not as characters but as players.

The most common example of this is simply being asked to roll initiative. If you are in the middle of diplomatic relations and you make a bad roll (or even a good roll) and the DM follows that up with "roll initiative," most players are going to interpret that as a statement that diplomacy has failed - even if that is not stated and technically nothing has changed in the game world.

I would recommend discouraging this kind of "anticipate what the DM is thinking" metagaming from your players. When things devolve into combat, make it very clear how that's happening in the game world before you ask for initiative. And let your players know that anything can happen at any time.

If I was in OP's situation and I got the feeling that the players were acting off metagame cues rather than in-game cues, I might prompt them just a little bit like, "How does Gronk feel about giving up Skullsplitter at the door?" Just to pull the focus back into the game and remind them to look at it as characters rather than players. It doesn't seem like much but I think this kind of thing can really change how the players feel and how they engage with the scene when the twist happens.

Hm. I failed to note the point about "My campaign opens" -- you can't ask the players to come up with a solution if you just placed them in the situation to start with, and unless they're starting at higher level, they probably don't have a lot of magic items anyway.

However, this is still something you can to a large degree up to the players. Just tell your players

This campaign starts at a royal wedding. Come up with a reason your PC might be at such an event; perhaps you're a servant or guard, perhaps a minor guest, perhaps something more dubious such as a thief pretending to be a servant to scope out the palace. You should not have any items that would not be appropriate to your role; most people will not be wearing armor or carrying larger weapons.

At that point, when the hostility breaks out, start that scene out without initiative -- there's stuff going on in the distance that doesn't directly involve the PCs. Give them a bit of time to figure out what they do (run away, find weapons, steal the silverware, w/e) and then get to the combat.

Quote from DruidofMN >>

Are there any established small easily concealable magic items that could get them ready?

 Secret Chest maybe? (a patron could supply it if they are low level)

Another option might be to have all attendees check their weapons at the door - like a coat check for weapons.  This room could be someplace not far from where they're at and easy to get to when the commotion starts.  That would at least give them a chance to dash to that spot and retrieve their weapons.

There are no low level items. As others have said it’s really the player’s problem to sort out. However,  keep Thorin Oakenshield firmly in mind - his name comes from him using an oak club as a shield when his was chopped to pieces. There are plenty of improvised weapons and armor(shields) available with chair legs for clubs and shields, lots of throwable ( plates, glasses, silverware etc) wall hangings and cloaks for nets, etc. realistically your casters should be standing back casting cantrips and L1 spells and only your martials would need armor and weapons. If the party has a Druid remember that shilllegh is NOT a concentration spell so your Druid will spend rounds nuancing all those chair leg clubs. 

Quote from Wi1dBi11 >>

 If the party has a Druid remember that shilllegh is NOT a concentration spell so your Druid will spend rounds nuancing all those chair leg clubs. 

 Only one problem with that plan:  The last line of shillelagh says: "The spell ends early if you cast it again or if you let go of the weapon".  But at least the druid will have a weapon.

Or one of the martials - the Druid has either wildshape or spells. Better to let a martial have the shillelagh and step back casting if unarmored.

Quote from Wi1dBi11 >>

Or one of the martials - the Druid has either wildshape or spells. Better to let a martial have the shillelagh and step back casting if unarmored.

 Shillelagh has components they may not have access to. 
But more importantly, as others said, it stops working if you let go. For the martial, it will just be a table leg. 

Quote from Wi1dBi11 >>

Or one of the martials - the Druid has either wildshape or spells. Better to let a martial have the shillelagh and step back casting if unarmored.

 The spell ends if you stop holding the stick.  Can't give it to another character.