Do buy her a good Christmas present. Now is not the time to break the news to her that because of your religion you do not celebrate holidays. Save it because she does celebrate holidays and will be expecting a gift. You should've told her that when you first met so if you didn't, regardless of any excuse you can think of, you are now obligated to get her a gift. In the same token, nothing is more embarrassing than having a nicely wrapped box with a wack gift inside of it. What you don't want is for her to open the box with a ton of anticipation, only to regret ever opening it once she sees what's inside. Take a week or so to pick out a few different gifts that fit her taste. As the week progresses, you can narrow it down to the one or two gifts that suit her best. Otherwise, she will spend the rest of the holiday trying to convince herself why she should bother keeping you around.
Do set aside some quality time for her during the holidays. Every girl likes to feel special. Women are emotional and the holiday season brings out their mushy side even more! Set aside some quality time just for the two of you to catch up amidst the hustle and bustle of life. This will give her a moment to catch her breath and the two of you can create new memories to reminisce on for years to come. P.S. Be sure to take a camera to capture those special moments. Pictures are worth a thousand words!
Do look nice when you meet her family. First impressions are everything. If you look good, she looks good. But if you look like a hot mess, it will taint her reputation of the types of guys she chooses with her family. Preferably, head to the mall and grab some new gear with some fresh kicks to set it off. But if you spent so much on her gift that you can't afford it, then put together a cool outfit from your closet. If in doubt, ask her to help you pick it out.
Don't show up empty-handed. Cook a dish, buy a dish or borrow a dish from your own family gathering. However, please do everyone a favor and taste test the dish first. Nobody wants to eat your non-seasoned or undercooked food. What are you trying to do, give everyone salmonella poisoning? If cooking is just really not your thing, then stop at the closest store and grab some beverages and a few bags of ice. You can never have too many beverages at a holiday event.
Don't monopolize the conversation when visiting her family. OK, this is very important. This is not your event. This is a family and friend event, which means everyone should get a chance to talk, not just you. They want to get to know you, but they don't need to know your entire life story in one sitting. Join in on the conversations to be polite and mingle because you don't want to seem standoffish or antisocial. If you look across the room and see one or more relatives nodding off in the corner while you are still telling your story, that's your cue to rush to your punch line. Zip it buster!
Don't combine her birthday gift with her Christmas gift. Quick history lesson: Christmas is Jesus' birthday not hers, and it's celebrated by giving gifts to those you love. Just because her birthday happens to fall in December doesn't mean you can give her one gift and say "Happy Birthday/Merry Christmas" all on one card. That is the epitome of tacky and chances are you will become a laughing stock for her and her friends. Do buy separate gifts and stop being shady man!