"Target attacking creature gets -3/-0 until end of turn" Help a noob out!
The -3/-0 of Elven Palisade is the effect of its activated ability. It's not something that happens when you cast the palisade (which indeed will usually happen on your turn).

You can activate the ability (and pay the cost of sacrificing a forest) any time you have priority. But (as noted in the comments by Gendolkari) you must choose an attacking creature as the target, so generally you will do this during the opponent's turn after they declared attackers and make one of the attacking creatures less powerful until the end of their turn.