We measure time by the way things rotate. It takes one day for the Earth to spin one full rotation on its own axis. It takes a little less than 28 days for the moon to rotate once around the Earth. It takes a little more than 365 days for the Earth to go around the sun. It would be nice if all this rotating was perfectly synced up, but it isn’t, and since we can’t control the motions of the planets, we are stuck with a messy calendar system that needs a bit of adjusting now and then.
Leap years are one way we correct for mismatches. Having months of varying lengths is another. In the calendar we use, the months are only roughly inspired by moon cycles. If we measured months by the moon and the year by the sun, the seasons would start to drift and eventually the Fourth of July would be in the dead of winter.
The Hebrew calendar, used to determine the dates of Jewish holidays, follows the cycles of the moon more closely. Months are 29 or 30 days, but complicated adjustments are made, including the addition of an extra month every so often, in order to ensure that certain holidays show up in the correct season. Hanukkah is supposed to start on the 25th of Kislev, which after drift and adjustment, ends up falling between the end of November and the end of December. This year it’s on November 28th, the earliest date it can be, which also happens to be Thanksgiving.
The 25th of Kislev will fall on November 28th again, but not when November 28th is also Thanksgiving (fourth Thursday of the month). Also, there is a tiny difference in the way adjustments to align the seasons are made in the two calendars, so tiny that they only matter after they accumulate for a few hundred years. By the time enough years have passed to get to the point where Hanukkah would fall on a November 28th that was also Thanksgiving again, that tiny divergence will have pushed Hanukkah over the line to the next day. And there’s no going back to the earlier date. The earliest date for Hanukkah will slowly keep getting later...
...unless another big adjustment by decree is made, which is what has happened every couple thousand years in the history of calendar keeping. If Hanukkah and Thanksgiving coincide again, it won’t be according to the systems as they are in place now, but through our improvised efforts to keep up with the sloppy, syncopated spinning of the spheres.
If you want more specific details on the adjustments, here is a more in-depth explanation of the numbers.