In the context of the Eurovision Song Contest, why is September 1st the start of the "New Eurovision Year"?

According to the Eurovision rules, September 1st is the start of a new year. This rule states that any song that is released after and first performed after September 1st qualifies to keep things fair. However, this is not a hard fast rule. If a song is played first but does not appear to gain an advantage the judges may allow it to qualify for the year. This is also true for a song that has been released on Spotify and had zero traction but suddenly gets discovered. The judges may allow it to also qualify even if it was written and released before September 1st.

The songs (lyrics and music compositions) submitted to represent the country of each Participating Broadcaster's country in the ESC must be original and must not have been released and/or publicly performed in part or in full before 1 September 2023 (the "Release Date").

In case the song submitted has been made available to the public, for example, but not limited to, on online video platforms, social networks or (semi-) publicly accessible databanks and/or performed publicly, for example but not limited to during concerts, prior to the Release Date the Participating Broadcaster must inform the ESC Executive Supervisor, who shall have authority to evaluate whether the song remains eligible for participation in the Contest.

In particular, the ESC Executive Supervisor shall assess whether such disclosure prior to the Release Date is likely to give to the song an advantage in the Contest vis-à-vis the other participating songs.

The ESC Executive Supervisor shall have authority to authorise or deny participation of a song which may have been available to the public as described above. The ESC Executive Supervisor shall consult the Reference Group in case of doubt.

https://escinsight.com/2023/09/04/the-problem-with-september-1st/
https://en.wikipedia.org/wiki/Eurovision_Song_Contest
https://eurovision.tv/about/rules