We strongly recommend artists release their songs on Fridays. Friday is the industry standard day for releases and when DSPs (Spotify, etc.) most commonly update their editorial playlists.
We also strongly recommend artists set up their release with their digital distributor where it releases at midnight in everyone’s time zone rather than just their own. This way, when an artist announces on social media or emails etc. that their song is dropping Friday at midnight, that information is true for every listener in their global fanbase of various time zones.
Avoid releasing songs on holidays weekends or in seasons that wouldn’t ultimately serve the song or the majority population’s seasonal interests (ex. releasing a Christmas song in February, or releasing a sad or depressing song in the summer.).
See more information at When should I release my next song?