Middle!
Just please NOT the beginning of the month! That would ruin the fun of a "sealed letter"! :[ I like the anticipation, but a month long anticipation is way worse. sweatdrop
I think the prices would change a little too. i'm not sure how though, either higher because some people won't donate if they don't like them, or lower because more people can donate if they do like them.
The thing is, I just want to have them low at first so I can buy them, then hurry up and inflate so I can sell sell sell!