The tax and revenue office sets a firm deadline for everyone who is obligated to file a tax return, and it is May 31 of the following year.
If you can provide a plausible ground (e.g., sickness, missing tax documents, workload, etc.), however, it is usually possible to extend that deadline up to September 30 of that year.
For this purpose, an email to the tax office providing the grounds for the extension and the new deadline is sufficient.
If you are not obligated to file but choose to do so voluntarily, December 31 is the applicable deadline. Keep in mind that voluntary tax returns do not have to be submitted by the end of the following year. Instead, you have up to four years to file.