Each filing status is attached to a specific series of tax rates. In addition, your filing status will determine your standard deduction. So, you gotta get your filing status correct to use the correct tax tables and standard deduction.
Filing status is determined as of the last day of the year.
To use the Single status, you must be unmarried or separated from your spouse either by divorce or by a written separate maintenance decree. My divorce was final on December 29th. I was married for the entire year except for two days. Surely, I can file as Married Filing Separately. The last day of the year is the controlling factor.