« on: March 22, 2013, 05:27:51 PM »
As it says in your instructions, this is a wyeast smack pack. The smack pack allows you to "proof" your yeast before brewing, so that you know your yeast are viable before pitching. The pack swells and you know your yeast are viable. Sometimes during shipping the yeast will be subjected to high temps and maybe kills it, so they want you to proof it before you brew. It seems that the "few days before" recommendation is to cover their a$$. If you wait until brew day and you're mashed in and find that the yeast is dead, you're sort of screwed unless you can get more yeast, and someone would be on the hook for a wasted batch of beer. This seems to be a related to the mail-order side of it, as they seem to assume the worst case, that you have nowhere else to get yeast on brewday if yours are dead.
If I smack the wyeast at all, it's 3-6 hours before pitching. I wouldn't want to do it days ahead of time and let it sit around. Maybe that's fine, maybe not, I don't know. If you have a local shop that stocks that yeast, I'd proof it on brewday. In all liklihood it'll be fine but if not then at least you can run out and get a replacement and not waste your wort. If you have noplace nearby to get a replacement, it might be best to take their advice and proof before you mash in.
As others have said, you may need a starter anyway based on your volume and OG in which case you'll know days ahead anyway when you get your starter going.