Why does my apple tree have blossom in late summer?

This is unusual but nothing to worry about.

This is what is happening. Next year's fruit buds actually form over the summer and normally stay dormant until the following spring. However if there is a cold-snap towards the end of the summer, followed by a warm sunny autumn, the tree may get fooled into thinking that it has just gone through winter and spring has now arrived. You might then find a small number of the latent fruit buds start to flower, and you have the strange site of blossom in late summer, possibly with fruit on the tree at the same time.

It does not usually affect the blossom the following spring.