Milliman just came out with an excellent guide to required year-end compliance issues. Rather than me reciting it, you can access it at www.milliman.com/uploadedFiles/insight/Periodicals/cab/pdfs/15-10_YEretire.pdf
Note that there are a number of notices due by either Dec. 1 or Dec 31. Your third-party administrators (TPA) should have informed you of these by now but check this list.
Remember that plan sponsors that have prototype plans (as opposed to customized plans) must have them restated by April 30, 2016. Most have already done this earlier this year, but if you are unsure, check with your TPA.
Also, the IRS announced earlier this year that beginning January 1, 2017 it would no longer issue determination letters so those with customized plans may want to adopt a prototype plan this next year.