Your song must be released and live on Spotify before the morning of your campaign start date or earlier. This means the earliest date you should select is the day of your release.
Please note, if your song is not live on Spotify, you will not be able to set your Spotify Track during the campaign setup. This means you will need to update your campaign with the Spotify track link once your song is published.
You will be able to do so after scheduling your campaign on your dashboard:
Please read this article below for more detail on setting your Spotify track.