No idea on the "double XP weekends". Those tend to happen.... "irregularly".
Um. If you're expecting a "calendar" of what happens when.... you won't find that in this game. Events etc are announced less than a week before they happen.
Around Oct 20th or 21st to Nov 2nd will almost certainly be the witches festival. September might see the undaunted festival or a Telvanni peninsula themed event.
They usually turn on an 'Explorer's Event' after the year's chapter event.
Said that the fixed 2XP events are Witches, New Life, Jester's and Jubilee so if on this occasion they do not turn on 'Explorer's' after 'Secrets of the Telvanni' then the next 2XP would be Witches starting [educated guess alert] October 19.
i think it's good because it forces those who are vet players in pvp who play below 50 instances only out of pvp by forcing them to level up OR it forces them to not play a few days so someone else can play below 50 who's yet to learn and gives them a fair chance. maybe? maybe it does that? if it helps they should triple the xp for below 50 pvp instances?