Harvest Moon Back to Nature Rival Events

Has anyone ever timed the waiting period between events for the rivals? I'm trying to get them all married, minus the girl of my choice (Karen). I know they're seemingly random, but what are the requirements? My event count is 2 for Gray/Mary, 2 for the Doc/Elli, 2 for Ann/Cliff, and 2 for Kai/Popuri. What time of day/season/day/year should I look for?