A fiscal year is 12 consecutive months ending on the last day of any month except December.
Unless you have a required tax year, you adopt a tax year by filing your first income tax return using that tax year.
See https://www.irs.gov/businesses/small-businesses-self-employed/tax-years
Of course you can always request that the brokerage firm issue 1099s for the 2020 tax year currently, but it is doubtful whether or not they will comply with the request; accounting systems for tax reporting statements are typically structured based on calendar years.
