Looking at things further, appears we always file around 6 months late.
Apr 30, 2015 https://upload.wikimedia.org/wikipedia/foundation/7/76/Form_990_-_FY_13-14_-...
Apr 30 2014 https://upload.wikimedia.org/wikipedia/foundation/5/5c/Form_990_-_FY_12-13_-...
So I imagine each year we apply for the extension.
wikimedia-l@lists.wikimedia.org