Say you're trying to spawn something like Safat, Chernobog, Bone Crawler, or Salt and Light.
There's no guarantee to the spawn, even if the window is up.
If you start spawning at 36 hours, the mob might not pop until 72 hours later. In this case, it's very likely that the party spawning the mob will never see it.
My suggestion isn't to give rewards for spawning, but to inform the player when the conditions to spawn the mob have been met.
This does not mean Player A uses Chocobo Porter in The Peaks on Monday, and is informed Wednesday when Players B, C, and D are spamming it for the spawn. Only the last player to fulfill the condition should receive the notification. This wouldn't mean that the mob has spawned either, just that the spawn conditions have been met.