This is literally Tax 101.

Please download a copy of IRS Pub 17, its free, for referencing these tax basics. 

Every new preparer, at a minimum, should have this on their desk.

https://www.irs.gov/pub/irs-pdf/p17.pdf

I agree with the others who say married filing jointly would be this. Also remeber it is what you are on December 31st that determines what you are for the entire year. If you get married on december

 31st at 11.59 pm then for tax purposes you are married for the whole year.

@Just-Lisa-Now-  I have told this story here, before.  About a dozen years ago I had a client who married in September.  His wife had two kids.  She went to an H&R Block office as she always did, brought along her husband, told them when they were married.  They prepared an HoH return, as they had in past years.  When she and her husband came to my office to prepare his return, they told me this story.  I couldn't believe it; came close to accusing them of lying.  I told them to go back to the Block office and get something in writing from them confirming that she could still file HoH, knowing that they wouldn't do it.  

A week later, they brought me the memo from Block asserting that it had been done correctly.  

That is why I would not reccomend my worst enemy to go to H and R Block.  Well maby my worst enemy I would.

For tax pre-planning purposes, just tell your clients who get married in October under all circumstances DO NOT MOVE IN TOGETHER UNTIL JANUARY 2.

Now, it's loo late for that.