Keats simple means that the sun is getting rather old at this time of year: autumn. This is a prime example of personification, where human qualities are attributed to animals and inanimate objects.

Taking this example one stage further, we might say that the sun was in the full bloom of youth during the summertime, when he shone for virtually the whole day. But now that autumn's arrived, the sun doesn't shine as often as he did back then. He's a lot older and so needs a bit of a rest after all his hard work during the summer.

That's not to say that the sun is completely idle. He still has an important role to play in conspiring with autumn to make fruit grow on the vines that curl around the eves—roofs—of thatched cottages. If the maturing sun wants a complete rest, then he'll have to wait until winter.