-edit- Double XP week
Confirmed as running until March 9th! https://twitter.com/Halo/status/1233493292194684930
Double XP seems to have gone live.
I just got 36k performance XP from Firefight: https://i.imgur.com/cur4ang.png
Season 2 should start when Halo: CE releases, not sure what they're going to include in the rewards, either way you can always go back and get a previous season's rewards, nothing will ever be locked away from you.
The next season won't start with the release of HCEA - Max has some additional details in the Feb development blog (which I'm told is now live on Waypoint). And you'll always be able to go back and still unlock content from any prior season.