In a nutshell
A company’s financial year is the period covered in its annual/statutory accounts. This is typically 12 months long, except for the first year when it can run from the date of incorporation until the end of the same month the following year.
For example, if your company was incorporated on 15/02/2020, your first financial year will be 15/02/2020 – 28/02/2021. But from then on, it will be 01/03/2021 – 28/02/<the-following-year>.