Robert Eisen teaches Judaic Studies at George Washington University. He says the Jewish calendar is based on the moon and has shorter months than calendars based on the sun.
"Well, we have a leap day, February the 29th, in the regular calendar, or the calendar that we're used to," he says. "Here, it's a month, a leap month that has to be added. But when the calendar gets out of kilter a little bit, you can have Jewish holidays fall much earlier in the year with respect to the solar calendar than people are used to."
He says Thanksgiving and Chanukah won't overlap again for more than 70,000 years.