It's best if you send out early, esp to overseas friends so that they have time to make arrangements. If u look at the preparation lists, the recommendation is to send out 6-8 months ahead, which makes all of us late already, if u have not sent out.
I feel that if you want to send in mid-March, then there is not much purpose, cuz there is no need to preinform them to save the date, since you will send out the invite cards in 1-2 weeks time and plans usually will not change so much within 1-2 weeks.