This came up in a pvp game and I could see it happening often. Just curious what everyone’s thoughts were. I can’t seem to find anything like this in any rules. 

Turn 1

P1 moves behind a wall with total cover. 

P2 readies an attack with a Javelin from a distance. The readied attack is for anyone coming within the range of the javelin. 

Turn 2 

P1 moves out from cover and into range of the javelin attack. Both P1 and P2 are in the open. P1 is going to take the dodge action for the turn. 

P2 will make his readied action attack from the previous turn. 

Does P2s readied attack have disadvantage from P1s dodge action or not since the readied action technically came before the dodge action. 

Quote from Kyleh2486 >>

Does P2s readied attack have disadvantage from P1s dodge action or not?

 Yes, if P1 can see P2, P2 has disadvantage.

If they moved into range before taking the dodge action the held action takes effect before p1 takes the dodge action. Therefore no disadvantage. In my opinion anyways.

Or can P1 take the doge action, then move into attack range to gain the disadvantage?

In that order it would work like that yes. But it needs to be before the readied action conditions are fulfilled to work.

Quote from Kyleh2486 >>

Does P2s readied attack have disadvantage from P1s dodge action or not since the readied action technically came before the dodge action. 

Unless P1 used their dodge action before conditions for the readied action were met, then no, P2 would not get disadvantage on their attack. Otherwise, the 'dodge' action would be as a reaction to P2's attack - which isn't possible.

Ultimately, the readied action resolves as soon as the requirements are met. In this case 'as soon as someone comes in range' - the moment P1 leaves total cover, there's a javelin that has to be resolved. A shrodinger's javelin, if you will - an attack that both has been made, and not made, depending on if P2's readied action criteria has been met.

From a DM standpoint, I'd argue that the conditions should be a little more specific. 'Anyone coming in range of the javelin' could be a little ambiguous. I'd suggest 'As soon as P1 comes out of cover.'

So it’s possible to take the dodge action at the beginning of the turn. Then move out of cover, opening myself up for the readied attack. Then the attacker has disadvantage on the readied action? That would be the best move then.

RAW, yes. 

Technically, if you're taking the dodge action because you know the other player has an attack readied... then that's a little meta, but that might be how it goes in your game.