I'm curious as well...I haven't heard about anything special happening on March 20. There is the mini-Gay Days weekend on Mar 18 and 19. I looked at IsItPacked and RHM Travel and both indicate that it's expected to be crowded. There are some school breaks, but I don't know why that wouldn't impact more days of the week. Curious!
March can get busy because its right around the spring break time. My fiance and I went for two days in March a couple years ago. It was very busy for sure but that didn't keep us from having a great time. My best advice is to get to the parks early. We were able to do a lot of attractions and things in the first couple hours of park opening (one day in Disneyland and one day in DCA). Don't be intimidated by crowds, you will have an amazing time!
Interesting, it's the only 9 on the calendar for that time period. I'm not sure why a Monday would be so busy, especially compared to the preceding weekend when it's not a holiday.