Other games have thought about this as well.
I like the league like format. Aka you have one week to aggree on a gametime with your opponent. This way, players can always play on a week end. And on a weekend one can play across time zones. Of course, sensible rules about how the players are to agree on a time are nessecary, so that one player can't just say: "I'll play at 1PM, 2PM or 3PM. Don't care that's the middle of the night for you."
Aside from that. Multiple days are preffered. Once a play day goes over 4 hours, it get's complicated. Eating, doing chores, sleeping (remember timezone difference)...
And if there are several playdays of 4 hour length, it should be done within one weekend. Because keeping several weekends free for one tournament isn't a sacrifice everyone is willing to make, so you get a lot of drop offs for week 2.