To expound on Sam2’s correct explanation, each tekufa occurs 365 days and 6 hours after the same one last year. In the year of a civil leap year (such as 2008), Tekufas Tishrei is on October 7th at 3:00 am.
The next year (2009) it is on October 7th at 9:00 am.
The next year (2010) it is on October 7th at 3:00 pm.
The year before the next leap year (2011) it is on October 7th at 9:00 pm.
If next year (2012) would have 365 days, it would be October 8th at 3:00 am, but since it is a leap year, it goes back to October 7th at 3:00 am, just like 4 years previously.
Therefore, 60 days from the tekufa is always December 4th, at either 3:00 am, 9:00 am, 3:00 pm, or 9:00 pm, respectively. But since the Jewish day starts in the evening, December 4th at 9:00 pm is considered the following day, December 5th. So in the year before a leap year, we (i.e. those of us whose exile is in ??? ????, as opposed to those whose exile is being lived in ??? ?????) start saying ??? ?? ???? on December 5th.
(We are using civil dates for convenience, because both the tekufa and the Gregorian calendar are solar based. This has nothing to do with the fact that the legal day starts at 12:00 midnight.)
To take the calculation further, this only works for dates between 1900 and 2099. But since the gregorian calendar skips a leap year in 3 out of every 400 years, that means that in the 1800’s they would start on December 3rd or 4th, and after 2100 it will (theoretically) be either December 5th or 6th.