It's not easy to make out, but the postmark on the right side of the envelope is from Napapiiri: the Arctic Circle. I believe this comes from the Santa Claus Village outside of Rovaniemi, Finland, which is just a few kms south of the Arctic circle. At the top of the postmark you can see Santa Claus in his hat, peeking out over the date. And in the wavy lines to the left you can see a reindeer. It's not very clear but it's there. The round postmark on the left is a Helsinki postmark applied a day after the Santa Claus postmark.