If you utilise the entire range of species available, you can manage to have rhododendrons flowering in your garden from just after Christmas through to August or later.
The last to flower, in late July and August, are R. Polar Bear and R. auriculatum, which can be in flower a week or two later. The main flowering time for rhododendrons, however, is in April and May. Around this time, the colours of a rhododendron garden will resemble a great burst of fireworks.
Some Rhododendrons may get confused and flower a second time in the autumn. R. Elizabeth is well known for this. Another is R. Yellow Hammer which can surprise us by it's free flowering between September and December. Some years we have plants available that are flowering for Christmas Day