From Sue's response to a similar question in 2009 I found the following
link with some stats:
http://nccsdataweb.urban.org/knowledgebase/detail.php?linkID=214&category=118&xrefID=2579
it seems it is quite common to get the extensions, and I'm assuming the WMF
did that again in the past years. But
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_-_Public2.pdf
Apr 30 2014
https://upload.wikimedia.org/wikipedia/foundation/5/5c/Form_990_-_FY_12-13_-_Public.pdf
So I imagine each year
Upon reviewing this 990 form:
It says May 9th 2016 on the bottom which I assume is the date of filing.
https://upload.wikimedia.org/wikipedia/foundation/2/27/Form_990_FY_2014-2015_-_Public.pdf
Publication was the 18th of May
https://wikimediafoundation.org/w/index.php?title=Financial_reports&type=