The Schedule is announced.
September 10: Leave Summerhall September 15: Meet Dornish at the Boneway, hand off Elia's remains. September 20: Daynes attack September 23: The king pays a visit to Starfall. (outcome tbd) September 27: Leave Starfall October 11: Arrive in Highgarden
This is pretty much the order of things, and you are all free to tag into the attack or jump ahead or whatnot as you wish. Just keep in mind where your characters are at and when.
Baratheons/Arryns: You're probably going to arrive in Highgarden a good solid 2 weeks ahead of the king since the attack and whatnot moves his schedule back at least a week.