Board Thread:The Quarters - Q & A/@comment-203.30.100.1-20160518220544/@comment-24672610-20160518222106

The days it's set to be due don't change based on whether or not you've checked it off. Some tasks have a more fixed set of days on which they can/should be completed, so that makes sense for those; your example is one where it's a bit annoying.

You could use the Start Date option to readjust when you get off schedule. Whenever you check it off on a day it's grey, set the start date to be two days later.