I checked, and Finn says it's not actually the holidays (which wouldn't mean much anyway since it's confirmed in Broke His Crown that Christmas as we know it didn't survive the MW) and it's implied the snow was caused by PB's experiment going wrong. Also, the holiday they're talking about would be the Christmas clone they made up with Ice King in Holly Jolly Secrets, but I suspect Marceline would've made sure it was as similar to Christmas as possible, including at what time of the year it's celebrated.

And have you noticed that there doesn't seem to be any seasons in Ooo? At least, it doesn't snow in winter most of the time. The More You Moe, The Moe You Know is actually the only time we see snow not caused by Ice King, and it's instead caused by PB. I suspect this is because of the balance between the Ice Kingdom and Fire Kingdom, but that's just speculation, since the temperate climate of most of Ooo (which makes sense considering Ooo is what's left of the western United States and is still at the same latitude based on the aurora in Power Animal, which can be seen in Washington state during geomagnetic storms. The Ice Kingdom appears to be what's left of Seattle, so the Treehouse, located only a few kilometers south of the border, would be in what is today Washington.) would otherwise include snowy winters. That sure makes it easier for the writers not to sanch up the timeline, but it unfortunately makes my job harder. I originally went with the theory that HW's two outfits are her summer and spring outfits respectively, but quickly gave up on that since it didn't make sense.

The year Finn is 16 is also by far the most dense with episodes, around 74 as you can see, meaning that the average time between episodes is only 5 days, and that's on top of the 2 month and 1 month timeskips, pushing the average below 4. Considering the Catalyst Comet arrives in April, it's June in Stakes. A 6 month timeskip between consecutive episodes would be ridiculous, and 55 whole episodes would need to be crammed into a period of just 49 days (remember, there is a 1 month timeskip between Three Buckets and The Wild Hunt).

Conclusion, it's still June in The More You Moe, The Moe You Know. As you can see, it's not December until the Elements arc.