An annual competition in Lancre, where any witch worthy of her name will join the gathering to meet, exchange gossip, re-establish who's who in the pecking order (hopefully bloodlessly), and be deferential to Granny Weatherwax.
Oh, and incidentally they compete against each other with spells and demonstrations of Craft. As magical duelling of this sort can proceed to mutually assured destruction in no seconds nothing, by general agreement if there is to be a focal point or a target of any kind for applied spellcasting, it should be the scarecrow Unlucky Charlie, who has since developed a mind of his own under the pressure of randomly applied spells continually reacting with each other.
All this makes for a gala day out and fun for all the family - local Dibblers are not slow to set up concession stalls for the day. It is the sort of occasion where wizards might be tolerated, if not actually welcomed - just don't invite the Dean, who will surely want to join in.
For as long as anyone can remember, Esme Weatherwax has won hands-down, and the only academic (or betting) interest lies in who is likely to come second. (Second-place is a position Nanny Ogg is perfectly happy with.) On only two occasions has there been serious trouble. Once where Mrs Letice Earwig attempted to persuade Granny to stand down, so as to give somebody else a chance. The unfortunate (for Mrs Earwig) results are graphically described in The Sea and Little Fishes. Secondly, this was the occasion for Tiffany Aching's final confrontation with the Hiver, in which she meets Death for the first time (in A Hat Full of Sky.)
Petulia Gristle won at this second event, by doing the Pig Trick - without a pig! This caused cries of "There are children here, you know!" and "Impossible!" but she did it nevertheless, to universal approval.
The phrase "Petulia Gristle won..." requires some qualification. Granny Weatherwax was present - and everyone knows she always wins the witch trials just by turning up. It is possible that by holding back, and allowing Petulia a chance to be the best of those who chose to demonstrate their skills, was (a) quiet encouragement of the new generation of witches or (b) a finely calculated insult to Letice Earwig - having proven her point at the previous Witch Trials, Granny is, at least on the surface, conceding that Mrs. Earwig might have had a point. (And by agreeing with Mrs. Earwig, but at a time of her own choosing, is just coincidentally delivering one of the most potently calculated insults which one witch may deliver to another). In any case, she and Tiffany had just performed a major magic in a life-and-Death situation, which in a quite satisfying way was witnessed by up to three hundred other witches. Maybe it is more accurate to say that although Petulia won the open trials, she still only managed the runner-up slot...