Usually during the late fall and winter, particularly from November to February.

I hope all is well.We have a STR in the town next to Ludlow and we were booked solid for the Winter (every single weekend and many weekday nights.)Check out our property "Honeypot Farmhouse" here.We're just coming out of "Mud Season" now where everything shuts down (restaurants, etc), but entering the Spring and Summer, we expect the rentals to pick up nicely.

She's willing to pay a premium for the month-to-month arrangement.I've never done a month-to-month lease before and I'm worried it might be a hassle, especially since I don't know how the market will be in the winter.

The holidays and winter season is always tougher to rent than spring/summer/fall, which is when the 100 days you referenced would have occurred2. 

And I don’t have many bookings for the winter holidays, but probably would pay overtime for Xmas Day or New Years Day, or would have the cleaners come a day later if I had no booking afterwards.

You will also find that the dates of October to March are very slow in the winter and often times people turn them into medium term winter rentals in order to keep them occupied. 

No one can time the market, the best advice in REI is winter against springs but even then if you want to just buy X amount per half of year, X amount per quarter, or X amount per two years--cool.

Quote from @Mary Winter: Hello BP community!

They stated my HVAC will actually run more efficiently without insulation - because the warm crawlspace air will rise in the winter.

I understand that the rental market can be slow in winter, but since then, they have only shown my unit once.