"Didn't we do this back in February?", you ask. Oh no, you see, both Month of Letters and InCoWriMo are both sort of home-grown letter writing celebrations. April is the bonavide month of celebrating the handwritten letter according to the U.S.P.S. (It's also National Poetry Month, National Honey Month, National Woodworking Month and National Fresh Celery Month. I made none of that up. There's a whole list here.)
There is some contraversy about the "actual" dates. Universal Letter Writing Week is the second week of January; old WPA posters have it in the first week of October. I even found reference to a National Letter Writing Day on December 7th. It doesn't appear as if anyone is truly "in charge" of these pronoucements. The first evidence I found of April being as National (Card And) Letter Writing Month is from a U.S.P.S. bulletin back in 2001.