It depends which colours you're after but the pre-order for the Spring colours - I know lunaboston had almond and lapis - which was for March 31/ April 1. There should be other colours but I emailed a few different retailers who weren't able to provide any specific details as to which colours/ styles they were getting. HTH!
Some colors (like Black and Lapis) are scheduled to ship 03/31 at LB but that rests on whether they get them from RM first (we know how reliable that is ). I only know this because I asked about these a while back. I know there is a seaglass Mini Mini but LB is not getting them.
There is also a Wine Mini Mini coming out at the end of May.