I just finished my third year in-game, meaning that the third Dry Season just finished, around day 210. I stockpiled a bunch of gunpowder and some coconades to go to the volcano and gather obsidian. On the 2nd or 3rd day of Mild Season I head off and climb the volcano. First I use a coconade to destroy an obsidian boulder and then I hear the indication from my player (WX78) that the volcano is going to erupt. I thought I might have caused this due to using a coconade and so I switched to gunpowder. Once day hit (it was night when I entered the Volcano) I started overheating inside the volcano. I was so confused so I decided to leave the volcano halfway through the day. When I left, the volcano started erupting and luckily I didn't get hit kiting circles around the volcano. I come to find out that it's no longer Mild Season but in fact Dry Season. I waited to see if it would go back to Mild Season the next day, and it didn't. So I'm currently stuck facing 2 back-to-back Dry Seasons.
Step 1: Finish dry season. Step 2: Head to volcano during the first days of mild season. Step 3: Get indication that the volcano is going to erupt once inside the volcano. Step 4: Leave the volcano to see that it's Dry season and the volcano is erupting.