Best season is Dec-Jan, but July-August come close as well due to the nature of my products. It is only my first year, but I expect that the same pattern will hold for next year.
Every month has been pretty consistant. My slow months are Nov-Dec every year I've been selling on etsy. Also if Etsy is doing some tinkering with the site it also can cause a dip. Gah! @_@
I've only been actively selling on Etsy for one year, so can't say as I have any seasonal selling trends to report, but I looked at my 2012 stats and April was my highest sales month. I was frankly surprised that November/December didn't beat April!