Sometime after the attack from Lord Tirek, Twilight sends her mother a letter explaining her current situation, devastating loss, and why she can't be in Canterlot to celebrate it in person. Luckily, Twilight Velvet is a very caring mare and completely understands her daughter's situation. Twilight Velvet then makes a reply letter explaining that everything is alright, and that she is very proud of her daughter--along with her friends of course.
On the upside though, there's always next year for a better Mothers' Day.

Image From deviantart.com