What happens when my buy to let fixed rate ends?

You move onto the lender’s variable rate, automatically, on the day after the fix ends. It is almost always more expensive.

Nobody rings to warn you. The first most landlords know about it is a larger direct debit, and by then you have already paid at least one month at the higher rate.

You normally can start arranging the next deal three to six months before the end date, and most offers stay valid for months, so lining it up early costs nothing. The date is the thing worth knowing, which is why Tofta counts down to it.