One of the best things about trading ATCs, is creating special cards for birthdays. Sending ATCs as a surprise birthday card has been great fun for me.
The first cards I sent out were primarily cards I had on hand. Now I create birthday themed cards for the person receiving them.
I think that the greatest thing is that they do not know that it is even being sent their way.
I hope that you get a chance to make someone’s special day memorable by sending an ATC as a random act of kindness.

