my christmas poinsettia still lives!

[southeastastra]

will it go red again!?

yes it should. Keep it in a darkish room in the cool.

when? it's happy on the windowsill atm?

I guess in the winter. They will last for years if you're careful.

You'll need to reduce its hours of daylight if you definitely want to get it in flower again by Christmas. They have to be fooled into thinking it's later in the year than it is.

Perhaps a north-facing windowsill, in the cool as BuzzingNoise says. I think the professionals put black bin liners over them to reduce the amount of daylight they get but that does sound like a lot of faff...