Comparative, superlative using "one of"

Both are grammatical, but they mean different things.

The first supposes that several days this month have been warmer than the others, and that today is one of them. For example, the temperatures on 2, 8 and 19 September have been 16, 14 and 23 degrees and the temperature today is 18 degrees. All the other days in the month have been cooler.

The second supposes that several days this month have been the warmest this month, and that, again, today is one of them. For this to be true, the temperature today and several other days must all have been, for example, 20 degrees, when the temperature on all other days has been lower.