Examples of release events include physical events such as The Voc@loid M@ster and Comiket, and virtual events such as APOLLO and the character birthdays.
Release events almost always have a date. Some events may last multiple days. In that case, the event date on VocaDB is generally the first day of the event.
Albums and songs can be connected to a release event. This connection indicates that the album or song was released during or for that event. For physical events this generally means that the album was first distributed at that event, so the release event date matches album release date.
For virtual events it's possible that the release date doesn't match exactly with the event date, but usually it's close. For example, for character birthdays the albums might be released a week ahead. In this case it's still acceptable to connect the album or song to the release event, as long as the artist or publisher officially states that the album or song was released for that event.