Flying Ant Day

It's here! Only it's not even a day, it's usually just about an hour. What's it all about? I left the house 45 minutes ago - nothing. I've just got back and the air is full of the horrid critters. Where are they all going, and why do they all grow wings and suddenly take flight all at once?

I suppose I ought to look it up on t'internet and tell you, oughtn't I...?