Now is the best time.
There are a number of reasons for this amongst which are;
As the fire has had fairly recent use the soot is still dry and can be removed better. It is always a good idea, if there is a delay between the last fire and the sweep calling, to have a small last fire a day before the sweep’s call, ensuring dry soot and a good job. Damp soot is more likely to be turned into mud and stick in the flue, eventually falling off when you light up next time perhaps leaving you with the impression that the sweep did not do a good job!
Damp soot left in the chimney for the summer months does no good to the structure of the chimney. Older flues with brick/cement etc. will start to rot as soot plus moisture produces an acid, which attacks the inside of the flue. Even the more modern stainless steel flues are susceptible to rotting from damp soot. Get it cleared out.
If the winter returns in July you will be ready!
If you wait until the Autumn you will then have to wait for the sweep. He/she is expected to sweep 2/3rds of the year’s work in 1/3 of the year, September to December.