Can you cast a spell during the end step after a 'until end of turn' effect has expired?

'Until end of turn' effects do not expire during the end step — they last until the cleanup step, which is the final step of the turn and comes after the end step (CR 514.2). This means any temporary boosts, keywords, or other 'until end of turn' modifications are still fully active during the entire end step.

The end step (CR 513) is a normal step where players receive priority and can cast instants or activate abilities. Triggered abilities that trigger 'at the beginning of the end step' go on the stack here, and players get chances to respond. All of this happens while 'until end of turn' effects are still in place.

The cleanup step (CR 514) is where those effects actually wear off. Notably, players do not receive priority during cleanup under normal circumstances, so you generally cannot cast spells there (CR 514.3).

Concrete example: Your opponent casts Giant Growth on their creature at end of turn, making it a 5/5 until end of turn. During that same end step, you can cast an instant like Lightning Bolt targeting the 5/5 — the +3/+3 bonus is still active because cleanup hasn't happened yet.

So if you're hoping to respond to a board state before buffs wear off, the end step is your window. Once cleanup begins, 'until end of turn' effects expire, and by then it is normally too late to cast anything.