You can plant it now, unless it is already in flower in which case wait until it has finished flowering. Remember needs acidic, ericaceous, soil.

It has buds but no flowers yet

It 'should' be okay but you might want to wait until it has flowered and then plant out in the garden. One of ours was flowering beautifully then we had a frost and now the flowers look like used tea bags... oh well next year.

You can plant it now, prepare he hole and give it a good start with ericaceous compost, keep well watered during long dry periods, once planted mulch with pine needles if you can get hold of some, this will keep the moisture in and protect just in case the weather turned and we had a severe cold snap.