Jewish holidays (eg: Hanukkah) follow the Jewish calendar. The Jewish calendar is based on the lunar cycle (with an occasional leap-month to keep the months/holidays synced up with the seasons.)
This means that Jewish holidays fall on a different date each year on the secular calendar.
If you want to know when the holidays are, HebCal.com is a really good resource. It has a list of when all the holidays are, and a lot of other information. It also has a calendar generator that can be exported to most calendar software including Google Calendar to make your regular calendar know when the Jewish holidays are.
Is it a Jewish Holiday Today? is a website that tells you whether today is a Jewish holiday, and if so, which one. It’s only useful for telling you if it’s a holiday right now when you look; hebcal.com is much better for planning ahead.
HebCal is glorious, but don’t try to use it to find out what specific time passover ends… I tried that last year, and spent way to long searching the internet for an answer.
Yes, that’s true. If you need the precise hours that a holiday begins and ends, HebCal isn’t necessarily the best site. The best site for that information is myzmanim.com.