Form 709 is an annual return.
You file that return the year after you made the gift and must file on or before April 15th (if April 15th falls on a Saturday, Sunday, or legal holiday, Form 709 will be due on the next business day).
For any gift made during the 2020 tax year which is required to be reported, Form 709 is required to be filed between January 1st and April 15th of 2021.
An extension of time to file is available.
See https://www.irs.gov/instructions/i709#idm140535812822672