Getting fuel on Christmas Eve and Day in Bavaria?

It can be found (a) on the Autobahn, (b) in unmanned automatic filling stations, where you put in your credit or debit card and type in your PIN.
But, beware, automatic filling stations require a "Chip-and-PIN" card. These are universal from European banks, but, according to many posts, many non-European banks have not yet converted to "Chip-and-PIN".
I think the rule would be if you pass an open filling station, fill up even if you are still half full.

Three hundred miles plus on a tank -- how far do you plan on driving in a day?

The fuel stations on the Autobahn network never close. Many of the stations immediately off the Autobahn are also never close.

One correction to what the previous poster wrote. Unmanned "automatic" fuel stations do not exist it in Germany. If the station is unmanned, it isn't open. And I've only once seen a station that offered pay-at-the-pump. Almost every other station will have an attendant working in the shop.

Good point on the mileage, but then I might go a ways and I want to make sure I can get back to Munich. I always like to fill up before I leave the town we visit, so just in case something happens I have fuel. Might be worrying about nothing and if we go to my cousin's for Christmas, then I have nothing to worry about.