Traditionally Astronomy Day takes place in the spring, sometime between mid-April and mid-May, to coincide with the first quarter moon around a Saturday in that time period. For 2021, Astronomy Day will be on May 15th.. For the next few years, Astronomy Day will take place on May 7, 2022; April 29, 2023; May 18, 2024; and May 3, 2025. Since 2007, a day to celebrate Astronomy also takes place in the fall, with the date falling on a Saturday close to the first quarter moon between mid-September and mid-October.