OK here goes, Mrrzy!

In England, possibly elsewhere, April 1st is April Fool's Day, when (mostly young) people play April Fool jokes on each other, mostly harmless little jokes like 'your shoelace is undone!'. If the unfortunate actually looks towards his shoes, he/she is an 'April Fool', and the subject of some derision.

Still awake? OK. 'flies' is a UK expression for the zipper or buttons designed to give easy access to a gentleman's nether reasons. So your 'flies are undone' is a practical joke designed to make you look and so be an April Fool.

Now this is where the story really gets interesting. April Fools Day in Hull (Yorkshire, England), and possibly other places, only lasts until 12 noon. After that it's Legging Down Day, when children endeavour to trip each other up (called 'legging down') by thrusting an ankle, often your own, in the way of someone elses. It's a tradition, of sorts

End of long boring explanation in the interests of improved intercourse between nations.