This would normally be the day that MLS rosters would have to be in compliance in terms of salary cap and international players, etc.
However, that has been delayed in light of the fact that some of those issues are covered by the new collective bargaining agreement -- which doesn't currently exist.
Here's the MLS statement released today:
"MLS is delaying the roster compliance date, originally scheduled for March 1, while discussions continue over related topics with the MLS Players Union. The roster compliance date will be set in the next few weeks."