I can't figure out from the wording in the player's handbook if a spell cast as a ritual must be immediately cast, or if the caster could hold onto it until it was needed. It doesn't seem to specify or imply either way.

I'm not using it as a feat, but as the inbuilt ritual caster as a druid.

Specifically, could my druid cast a ritual spell when she wakes up in the morning, to have it ready to go later that day? 

Rituals are cast in the moment. Their cast time is the cast time of the spell + 10 minutes. Once the cast time is done the spell goes into effect.

So unless the spell has a very long duration no, you need to cast it as and when you want to use it.

Ok, thanks!

Which isn't to say that you couldn't be traveling while you are casting it. If the party visited a place that had languages written that nobody knew and left only to find out that they had to return, you could cast comprehend languages on the way up so that your wait time didn't have to be the full 10 minutes. 

For all intents and purposes, @Emmote is correct.

Ritual casting just increases the casting time so you don't have to use a spell slot. So you can ritually cast a ritual spell and then at the end of the ritual cast the spell and ready an action to concentrate on the spell before releasing it. Now granted that only gives you an additional 1 round to release the spell, so it is "essentially" wanting to cast it when you want to use it...

You can't ritually cast a spell and concentrate for 10 hours holding the spell, for example