One way to approach this is just through the local temperature anomaly in and around Moscow.
Weather isn't really Gaussian. Physical constraints truncate the tails of temperature distributions. So we can ask this as a relevant objective question: has a prior calendar month on record anywhere ever been 4 standard deviations hot or cold?
Update: There is a 30 day period in a place called Lukojanov that seems to be pushing 6 standard deviations.
That's handy because we can refer to the industrial literature, where "six sigma" is a commonly stated ambitious goal. If the distribution were Gaussian, this would happen 3.4 times per million samples, or once every 300,000 months or about 25,000 years. Maybe there are 250 regions on this scale, so generously we have a slight chance of matching this on the observational record, and a pretty good chance of a historical match. But as I said, that's probably a huge overestimate, because I don't think the tails are occupied.
But, it becomes an objective question.