This year's "Fête du Salève" (Mont Salève en Marche) is at the other end of the mountain of the same name.
https://www.rando-saleve.net/pdf/montsalflyer2019.pdf
http://www.syndicat-mixte-du-saleve.fr/index.php/animer/evenements-sur-le-saleve

We will meet up at the end of Bus 44 at Croix-de-Rozon-Douane at 09:35 h (take Bus 44 from Carouge-Rondeau at 09:26 h). From there we will walk up via Collonges and Le Coin to La Croisette and then across to the fields of Chavannes, La Thuile and Les Convers. Return to Collonges by the same route, or via les Molliets, Chez Favre and Vovray (shorter but has a few steep bits near the top). Distance 21 km with about 700 m altitude difference; total walking time about 7 hours.

You need your hiking boots, hat and sunscreen, rain protection, your passport or ID (we will be in France), some Euros, water and a picnic. Reasonable fitness required.

We are some of the regulars of David's group (Association genevoise des amis du Salève, AGAS) that walks up the Salève every Sunday (see one of his posts here: https://www.glocals.com/things-to-do/geneva/106310.htm). The main group will still meet up at Veyrier at 10:00 as usual.

No need to sign up, just turn up. Expect to be back in Croix-de-Rozon for the 18:18 h bus (or, if we should linger at the "Fête" for too long, the one at 19:24 h.)