I find that many people call it extensions season, but the term isn't as widespread as tax season for the earlier part of the year. I've run into other people who lump both into tax season. Depending where one stands in tax, the post-10/15 period is planning season, sales season, or even busy season for certain parts of the tax accounting method change industry and for almost all of the M&A world.
-Brian
Director of Tax Accounting Methods & Credits
SourceAdvisors.com
Opinions my own.