HydroGuy has several threads that might also be useful to look at. In his Compilation thread, check out the threads in the Seasonal at DLR section.
http://www.disboards.com/threads/hydroguy-tips-compilation.1520483/
Easter is in late March in 2016, so you're going to be missing Spring Break (and the associated crowds). Memorial Day has (historically) been the kick-off to the summer season, which generally means longer hours, more entertainment, and higher crowds. We do know that April 30/May 1 will be the Spring Dapper Day. The Tinkerbell Half Marathon is likely to be in May in 2016. Also, in May and June, there may be grad nites (schedule is not yet known).
You can look at the 2015 Calendar of Events to get a sense of what kinds of things happened this year. Note that all events can change, so just because it happened this year, doesn't mean it'll happen again in 2016.
http://www.disboards.com/threads/2015-calendar-of-events-school-breaks-and-dlr.3358090/
If you want to see the fireworks, you can plan a Thursday and Friday, crowds will grow Friday evening (lots of locals come sometimes just for fireworks or to squeeze in a few hours) but won't completely explode (out of season).
Just to clarify, but during the non-peak season, fireworks are typically only presented on Friday, Saturday, and Sunday nights. F! also runs on a reduced schedule during the non-peak season. We do not yet know what the schedule for PtN will be, but WoC has (historically) run nightly.
HydroGuy has a thread that allows users to easily link to the wdwinfo DLR calendars to see what the hours, entertainment, refurbs, etc. were for a given month/year.
http://www.disboards.com/threads/historical-dlr-park-hours-entertainment-and-refurbs.2291124/
Just know, Spring is high season for ride refurbs, though they can happen any time. Keep an eye on the calenders here and at the DL site.
I don't think Spring is necessarily any worse than any other season for refurbishments. There are some that happen on a fairly regular basis, but this year (2015) was kind of an anomaly as the parks got ready for the 60th anniversary. If any season would generally be considered a "high season" for ride refurbishments, it would be winter because of the typical water ride closures, the overlay removal, etc.