Let's say before I even enter combat, I cast Blade Ward as a ready action, and saying "I'll release this as soon as Haste ends," then cast Haste on myself
After Haste ends, it says that
When the spell ends, the target can't move or take actions until the end of its next turn.
Apparently, you can still take reactions even after Haste, is it the same for a readied action?
An example for a sequence of events in case my description is confusing:
Let's say my party enters a potentially dangerous area, so before that, I'll cast/prepare Blade Ward as a ready action, stating that I'll release it for after Haste ends (but I don't cast Haste yet).
Once we enter a place with enemies (and we're not ambushed), I cast Haste on myself. Let's say after some turns of combat and my turn is over, Haste ends, and I'm left not able to do anything next turn as per Haste's effects. During that turn (when I'm not able to do anything), Haste has ended so Blade Ward should be released/take effect (if it's possible)
Best Answer
It does not work, but not for the reason you think
Two concentrations
Readying a spell needs concentration, so your readied Blade Ward end the moment you start casting Haste (which also needs concentration).
Ready lasts only until your next turn
Player's Handbook Errata:
How it could work
If someone else cast Haste on you, you can cast Blade Ward from your normal action1 in the round before you expect Haste to end.
1) not the one gained from Haste