What Year Card?

When you are playing a decades season are you playing the players average over the decade or does SOM pick, without you knowing, one year? Is there any way to know before the season ends?

One year, and you don't know which one (in fact, those leagues are called Mystery Leagues.) But there are ways to try to guess which season you have, and there is a huge thread on the subject...
viewtopic.php?f=9&t=634688

Wow, interesting. Thank you.

You're welcome. Good luck!