@Kent Lew you make a strong point but I hesitate to lump December in with summer. The thing about December is that we do always see a jump in traffic in the first half because there are definitely companies that are closing out the fiscal year and need to make purchases. That's a massive contrast to the deadness of summer. If you release in December you will get SOME eyeballs. Plus, you're not likely to be one of many foundries releasing that week/day which definitely sucks. So, less than ideal but not the black hole of July and August. The proximity to January (resulting in a low cost to wait) makes me want to agree with you and put a red X over it but I'm going to stop just short of that.
I tend to think the important variable is where your customers are. For folks like us who have a global customer base with the largest block (but still under 50%) in the US it's January.
This is because the ideal is to have a few uninterrupted months where the font can get some momentum. September is less than ideal because people are catching up from summer in the first month and you really only get October before Americans have Thanksgiving and the everyone is scrambling for the end of the year plus taking off for Xmas and New Years.
Launching in January means you get 5ish uninterrupted months where the majority of the customer base is just working (not closing out the year or prepping for a holiday or whatever).
That said, you will notice that we didn't launch Halyard in January but instead in May - along with 5 other foundries that week. The best laid plans.... Ultimately, the best time to launch is when the font is done - with the exception of July and August when you shouldn't launch.