On your 2019 return you'll be taxed on the wages shown on your 2019 W-2. Ask your employer's payroll department if your year-end wages are going to be reported on your 2019, or your 2020 W-2.
By the way, your question illustrates why no one should prepare their tax return on the basis of their year-end check stubs. Always wait for the W-2.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
