I'm not exactly sure what you mean by hardest! I'll list a few from my varying perspectives and see what you think? :)

September - In September, currency can come relatively slow, and GMO cows and gnawrocks aren't cheap in the shop. I don't grind September properly though, with timers and lambs, so this is my experience playing somewhat casually.

February - I think February isn't exactly hard, but if you're intense, you're going to go crazy. Timers every 15 minutes, lambs going constantly, and so much exploring. February currency is pretty plentiful in explore, and I grind excessively, so it's not hard for me gameplay wise. Just sanity and in terms of preparation. Getting all the energy items can be expensive.

October - Might be a little hard for new players to get the hang of. The best battle encounters for currency are a night, so imo it's only worth exploring during that time. This is easier with lambs since your energy will be full for every nighttime if synced properly, but again, the item preparation can be expensive.

August - this current month isn't that bad imo. Doing well in the minigame means you can get a lot of dry bones. I'm playing casually and score a buffalo scrotum every other day. Since it's the off month, they're expensive in the shop, but that's to be expected.

January - not hard at all, but you might also go insane if you play it intensely and have timers going off every 15 minutes

May has lion scrotums, but I never really bother and just get the decor. Probably wouldn't be THAT bad, but it's off month expensive.

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September is probably the hardest, the currency is not very numerous in explore, and it has several high in demand breeding items sold not found in other events I advise saving all your Roasted Lambs for September.

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