General release schedule:

Birthday scents (if they happen): September

Yules: October

Anniversaries: November

Lupercalias: Jan.

the Irish scents: March

April Fools: March/April

summer scents: May/June

Halloweenies: August

lunacy: each month

Friday the 13th: when it happens (May 2011, Jan, April, July 2012)

surprises: when they appear!

Note: I copied this from a post in the BPAL speculation thread, than added the next upcoming dates. I didn't come up with this myself!