There are 31 days in March.
31/2=15.5 so expansion could be March 15 at 12:00 PM.

Or...

According to Doc's thread the expansions have arrived on Wednesday, Thursday, and Friday before and the closest to the exact midmarch is on a Friday, 3/14/14. (Pi Day)

Or...

The expansion may arrive on Monday 3/10 right after the double odds Elusive Four weekend ends since there are no more events planned according to Remiem's thread.

I think the most likely day the expansion will come is 3/14