We've currently got a policy about which holidays are important enough to Drupal's userbase that we don't do an advisory on a Wednesday near those days, but this policy didn't get a ton of discussion before being agreed to.
The dates are:
- Thanksgiving in the USA (fourth Thursday in November).
- December (Christmas/Hanukkah)
- The end/beginning of the Gregorian year in general (i.e. around new year's)
I think we can all agree on Drupalcons, but what about the other dates? Are there any we should add that affect a large percent of Drupal's users? It's worth noting that Thanksgiving is the biggest holiday in the USA and always falls on a Thursday, so even though it only affects the USA it feels like a reasonable holiday for us to avoid.