You should smack it 3-24 hours before using. If you are going to make a starter you can just smack it and dump in in your starter.
I always make starters for mine, I think you end up with even better beer that way. But you do not have to unless you are making a big beer or lager.
My general routine with Smack packs is to smack it Thursday morning, make a starter Thursday night, then pitch on Saturday (brew day). If I am making a big beer, I'll smack on Wednesday morning and make a starter Wednesday night.
And now we go AG!
On Tap: Nadda
Planning: Extra Special Bitter