if you follow this link, you'll see when the window for the fate/hunt is opening/closing. Odin, as well as Behemoth, are normally only spawn able right after maintenance, the window if that hasn't changed, should be open for 36 hours. In that time, you can try "force" spawning these two special fates, however it's not a 100% chance, and when there's no weekly maintenance, then the window is on other days, since it varies by the time your server, killed it or spawned it last. If that window closes, you'll have to wait till it re opens, mostly 4-7 days, again it moves up or down a day, depending on maintenance schedules/last kills etc.