Leaf hasn't been to my island or my friends island in a long time! Does he stop coming in the cold months?
Leif just came to my island today (autumn for me) so I don't think seasons affect the arrival of special NPCs, and I think it's mostly luck-based. I've also had other "unlucky streaks" where a specific character didn't come to my island for a very long time. If this happens, you could wait until Leif comes or just time-travel. There are also some handy websites that explain the schedules of the NPCs. Hope this helped :)
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