Many New Rule cards allow for the opportunity to optionally do something “once per turn”. Many Keepers also have special abilities which can be invoked “once per turn”. Is there an official start and end to one’s turn? How do we know when a person’s turn is over unless they specifically state it?
A: A player may utilize any optional Free Plays as early as before their first Draw to after their last Play. Yes, officially a person’s turn is not over until they have taken, or declined to take, any optional Free Actions available to them, but it’s easy to forget one of these opportunities and let the next person start their turn.
Once the next person has started their turn, your turn is officially over, and you have lost the opportunity to use those. You can’t force them to undo cards they’ve drawn, played, or utilized. For very nitpicky players, using an official turn token can be useful. Your turn ends when you pass the token to the next player.
(However, we would rule that if you were playing with a Turn Token, and were lax about remembering to pass it, it’s up to you to enforce not letting the next person start their turn before they’ve received the Token. In other words, if you’re going to be rigid about the Turn Token, you have to actually remember to pass it. Otherwise, you fall back on the good old “Once the next person has started their turn, it’s too late…)
Keep in mind that if you forget to take a Free Action, it’s not the other players’ responsibility to remind you. Someone could be nice and remind you but they don’t have to. They might even pointedly ask “So are you done with your turn?” without specifically reminding you, though you might realize the significance, and double-check your opportunities…
Here’s a great chart with an official description Order of events in a Fluxx turn.