Filing of a 2020 tax return cannot start until after January 1, 2021. However, the federal forms for filing the 2020 tax return will not be available until mid-January.
If you are an employee then you must enter your W-2 on a tax return before filing, regardless of which tax year your are filing.
Employers have until January 31 of each tax year to provide you with your W-2. For tax year 2020, since the 31st falls on a weekend, employers have until 02/01/2021 to provide the W-2.
You could not have filed a 2019 tax return in December of 2019, just like you cannot file a 2020 tax return in December of 2020.
