Anytime now as long as the ground is not frozen solid. They prefer acidic soil (most of the UK is) and buy an acidic feed for it. Semi-shade is perfect.

i also have some to go but garden under 12cm of snow!!
i will probably leave mine until march/april now and put them where i can see them on the back path in flower.

I think I'd hold off now until March, unless it is bare rooted in which case you have to get into the ground.

If it was bought from an undercover part of a garden centre, or from a supermarket, it will probably need to be hardened off before planting out, so I agree with sbg and mg, wait until spring, but if you keep it in a warm place, harden it off first, Derek.

It is Azalea japonica, not an Indian Azalea, it can go out now.

I would still keep in the pot until March sorry...