Looking for a word for the point in time where one day becomes another

Solution 1:

If any kind of digital clock or calendar is in the picture, then rollover can describe this.

It needs a little context to make it clear: e.g. something like “Nominations open Saturday February 21, at the rollover” may not be understood by all readers, but “Nominations open at midnight, on the rollover from Fri 20 to Sat 21” should be well understood, as should “Nominations open Friday night, at the rollover from 11:59 to 12:00”.

Solution 2:

I think that at the stroke of midnight is a useful expression that comes close to what you are looking for. It actually indicates the last moments of the day and the beginning of a new one.

• the sound of striking, as of a clock

Solution 3:

You might try this :

On the cusp of a new day.

On the cusp:

• On the threshold or verge of a development or action. (TFD)