Coincidentally Edward Mayer, founder of Swift Conservation, posted this today. If you are using an attraction call player there is some good advice:
Some swifts are already back in their nests and they will
hopefully be back in numbers during the first couple of weeks of May,
then more, juveniles, will be arriving in June. The two best times to
attract Swifts are in May, when returning breeders may find their old
nest gone, and have to make a new one, and in July, when young Swifts,
yet to breed, try and find a nest place for the next breeding season, in
the next year.