In law school admissions we often refer to wait list movement as "summer melting" – which is a rather accurate appellation, for as the summer progresses so too does the melting of your admitted/deposited class. As schools ranked above you go to their WL to firm up needs (and needs could be any of a number of things, e.g. LSAT median, GPA, gender, diversity, great applicants etc.) you lose your admits and deposited applicants. It happens in true, albeit extended, waves. HLS, for example, will admit a batch of people off the WL and thus schools around HLS will lose a percentage of that batch, and will have to take their own group and so the dominoes fall. Generally this happens with large waves early summer (April and May) and then smaller waves in June and July. Starting in July and August you also see a wave of schools starting to take select applicants who they simply really want to admit (and often LSAT/GPA independent). It has happened like this in every summer that I've been a part of this landscape except for once, last year. Last year, for reasons I have blogged about, you saw hardly no movement at the top. When that happened the waves never rolled/the dominoes never toppled. What about this year's cycle though (2018/'19 cycle)? What can we look for this summer as far as melting?
First, the good news. There will be WL movement. Hooray! In fact, there may be a good deal of it. As of today, LSAT scores 165+ and above are all down. That itself is reason enough for there to be melting throughout the summer, and was the primary driver for the freeze last year. More schools have seen a decrease in applications than an increase. That too plays into summer melting. Schools might not have huge reserves in their applicant pool. The slow pace of the cycle may also mean that schools have not front-loaded admits (and thus actually need melting). All of these play into the likelihood of a good number of WL admits going out in multiple waves. But, of course, there is a catch.
The movement won't be as extensive as suggested above if the slow cycle has schools waiting on March scores simply holding applicants because they see a large retake number from their pool. March had a huge test-taker registration number of 20,000+, and there has not been a March LSAT before—meaning it is hard to make definitive predictions until we see the March numbers on April 20. That is a big date in all of this, and equally important is what happens soon thereafter in early and mid-May. There also exists a scenario where we have a new normal of small class sizes, which would mean there might not be continued WL waves throughout the summer.
Final analysis, I think there will be movement and it's going to happen in May. It will be more prominent than last year, I'd bet a lot of money on that. Whether it extends beyond May in large waves, though, is less certain—which for practical purposes means, if I were an applicant on WLs at schools important to me, I'd be putting my best (note best does not mean most persistent, most gift-bearing, or most dressed up as a lobster visiting an office – true story) foot forward in these next two months.
Best wishes for the melting!