Before Christmas. Have a clear out first. Challenge the boys to see if you or they can come up with most stuff for the charity shop/tip
After Christmas!! Have a family "camping" weekend sleeping downstairs on airbeds with sleeping bags while you move all stuff into one of the rooms and decorate the other. Arrange carpet fitting for Monday morning. Do the same the next weekend with the other room. See if you can get friends/relatives to mind the boys while you decorate. Even better can you send them to Grandma's for one of the weekends?
Once you have both rooms done you can decorate and re-carpet landing as this doesn't usually involve moving much stuff!
If you are having the same carpet throughout, ask at the carpet shop as they may still be happy to deliver and fit different rooms on different days.
Sleeping downstairs is a great idea! Hadn't though of that and it will mean that I won't have to get the rooms to a semi presentable state each evening.
We are putting the same carpet throughout all of upstairs so I will def ask about different delivery and fitting days. We have a third bedroom with dd in so there's no way I can empty the whole of upstairs to have a carpet fitted.
Decluttering is something I struggle with because we seem to need so much of it but since we are going from a very large double to a small double, we have no choice.