If you live in the United States and are an employer. The IRS release preliminary withholding tables for 2018 during the second week of January. Income tax laws were changed by the Senate and House of Representatives and signed by the President in the later part of December 2017.
The new tables should be in place by February 15, 2018. It’s ok to use 2017 for federal income tax withholding on employees until that time.