There was a flurry on 2/13 - the first day of pitcher/catcher workouts. I checked the Rules; Rule 2(g)(3)(F) states:
after the conclusion of the championship season, a player may not be placed on the 60-day Disabled List earlier than the first date that Clubs may invite injured players, pitchers and catchers to attend spring training workouts in accordance with Artlcle XIV(A)(1) of the Basic Agreement.
This is from the 2017 version of the Major League Rules (I didn't print out the 2018 version, to conserve paper).
I also checked a bunch of prior-year transactions, and these 60-day DL assignments began in mid-February as far back as 2009 (as far back as I looked).