I hate to dredge up an old post, but as you touched on a point I had made before -- my experience with seasonals is directly opposite of what you're suggesting. In the couple of months run up to the holiday and the month after, sales of seasonal decor and items in flea markets has been huge for me, year in and year out going back to 1999, and in a variety of markets. ie. Christmas in October, November and January, but not December. The problem, especially with Christmas, is that big retailers push all of their stuff closer and closer to the cash registers as the holiday gets closer, encouraging impulse buys to help them clear out inventory. So when people are doing their weekly grocery/Wal-Mart run they also pick up that item, saving a trip and often at a good price to boot.
One of my good friends runs a Christmas-year-'round retail store in a great traffic location. He's been there for nearly twenty years and has always maintained that the worst time of year for him has always been the month between Thanksgiving and Christmas.
I'm sure there are exceptions, but it's always been money in the bank in my experience to buy like a Rothschild right after the holiday, when prices are at their absolute lowest, and mothball the goods until before the season begins. This has been especially true with Halloween costumes and decoations and Christmas decorations.
Speaking of seasonals, an experiment we tried last year that might be worth repeating for some in the US: after Independence Day 2008 we picked up a ton of 'patriotic' stuff, paper plates and napkins and streamers and disposable flags etc. etc. etc. for super cheap from party stores, stored them and then whipped them out just ahead
of the 'patriotic holidays' in the US (Presidents' Day, Memorial Day, Independence Day and Veterans' Day) in 2009. This did pretty well on the local level; I wish we'd had more inventory to do a better sample though. But the merchandise in question is cheap and not prone to degrade in storage, so perhaps worth giving a shot this year for someone.