I know that there is a tax advantage to having a child on December 31st at 11:59 as opposed to January 1st at 12:01. Does the same hold true for marriage? If it matters I make ~$130,000/yr and have a rental property and a home, and my girlfriend is in school so is making -$40,000/yr and has effectively no assets.
It's easy to get a rough but practical answer for this yourself. As an exercise, do your taxes now, estimating income or other factors through the end of the year, as if you were both single. Then do the same, but married filing jointly. Compare the results.
I was married last year (for the 2nd time) and did this comparison "for fun" (it didn't impact our choice of date - I was just curious about the difference). We ended up with an advantage of owing roughly 4% of our AGI less than if we had not been married and had filed individually. That's a significant amount of money! Of course, your mileage may vary, but since filing status is determined based on your standing as of December 31, and marriage generally results in a tax benefit, getting married on the 31st means you can file as married for that tax year and take advantage of those benefits, for the entire tax year.
Generally there are tax benefits from filing as a married couple, specifically if one spouse has a very different salary. It can pull you into a lower tax bracket overall since everything (brackets, deductions, etc.) is roughly doubled, so one spouse gets the benefit of any "unused" margin that the other would not get to use as an individual.
To paraphrase JoeTaxpayer's catchphrase - "Don't let the tax tail wag the marriage dog". Marriage is a serious institution (at least it is for me). Don't rush into it just to get a little bit of a tax break.
Having just done this, the tax benefits are minimal. We got married in December (to fit our schedules, not taxes), and have a big gap in salaries.
I think we got around an additional 2-3% of our combined income back in taxes, but it's only due to the split in our income (split 13%/87%) and the effect of progressive tax rates.
Overall, we saved a lot more by finding a good venue.
Certainly run the numbers both ways to get a rough idea. Would you hit the state and local tax deduction limit ("SALT") of $10K on your own? If yes my guess is that you're better off not being married this year since you'll be able to itemize, but maybe not once you're married due to the SALT limit staying at $10K for both single or married.