2 Answers

Generally speaking, you can file a tax return as soon as you have all the relevant documents needed to prepare your tax return. This might include such documents as Form W-2 reporting wages and tax withholding, Form 1099 reporting bank interest, unemployment benefits, and other types of income, and so forth.

Some people who's only income is from freelancing or farming might be able to file their returns sooner, as long as you have tallied up your income and related expenses for the previous year.

One word of caution: try not to file a tax return using the information from your last paystub of the year. There's three reasons why. First, your last paystub might not reflect exactly what you earned for the entire year. Second, your paystub does not include all the relevant information needed for the IRS to process your tax return (for example, the employer identification number of the business). Thirdly, the IRS is on the look out for people who file only using their paystub.

Depends on the country in which you live.  Tax laws are different in various countries of the world.