You can decant a plant from a pot to the ground at anytime - now is a good time assuming the Acer is already outside.

thanks kildermorie its been outside in a pot for a few years so i can just plant it with sum garden compost and it will be ok?

As you have stated its been in the same pot for a few years I would expect it to be a bit pot bound. Ensure you tease the roots and plant at the same level as it was in the container.
Do you know what type of soil you have in your garden?

I use ordinary garden compost. They prefer but do not need acidic soil.

Great thats what i need to know ,iv been mixing in ericaceous compost and that seemed to work with another acer but i wasnt sure . iv got two great big camelias growing so would that mean iv got ericaceous soil thanks for the info its very helpfull

If your camellias are doing well then yes. They prefer an acidic site.
I always mix in some ericaceous compost when planting acers.
Although as Kildemorie states the will grow in neutral.
Good luck