In general, the weekdays (Monday to Thursday) are usually the least crowded days during the week. DCA has Extra Magic Hour on Monday, Wednesday, Friday, and Sunday for on-site guests, which means that when the park opens to the general public, things like RSR and TSMM already have long(ish) lines. DL had EMH/Magic Morning on Tuesday, Thursday, and Saturday, which means the attractions in FL and TL will have lines when the park opens to the general public.
In terms of April, the only even that I've heard about so far is the Star Wars Celebration at the Anaheim Convention Center on April 16 to 19th. Easter is on April 5th and Spring Break-season crowds are likely to be present through the first half of the month. If you're aiming to avoid crowds, I would look into going during the later half of April, as late into the month as you can.