We seem to have two parallel threads on the same thing here .....
http://www.wildaboutbritain.co.uk/fo...nt-swarms.html
When I lived in London I always expected the black ant nuptial flights at the end of the second week of July - indeed, for three years in a row in my locality they irrupted on Swithin's Day - I thought I was going to be able to write a 'rule'
but it's quite variable.