Why not just... let the festivities spread? These people/cultures/player characters have been interacting with each other for some length of story time now just have an Irish bar- - er Charr bar host some stuff. Have the Sylvari excitedly putting their own spin on other cultures' festivals and run that at the time of the main festival. Like a festive side-quest. Is that not how holidays operate in reality? Samhain and Halloween? The different version of Christmas and Yule?