Naimittika Pralaya happens “during the process of and after during completion of” one day of Brahma i.e. one day-time or 12 hours at the pristine abode of the creator (which in the Sanskrit language is also called the Brahmaloka).
And since one day-time or 12 hours at the pristine abode of the creator (or Brahmaloka), is named Brahma Kalpa, this type of Pralaya happens after the completion of a Brahma Kalpa.
When “12 hours of day-time of creator or Brahma (i.e. Kalpa)” gets completed, then “12 hours of his night time (i.e. Brahma Ratri)” starts.
Naimittika Pralaya happens “closer to and during” this night time (or Brahma Ratri) of the creator deity who is addressed as Brahma in Vedas … Brahma is the creator deity & is also addressed as Pitamah which means grandfather of allness.
During the stages of Naimittika Pralaya, the Milky Way Galaxy gets destroyed as other galaxies collide with it.
During Naimittika Pralaya there are three separate collisions of three different galaxies to the Milky Way Galaxy.
The duration of Naimittika Pralaya which happens during a Brahma Ratri (or 12 hours of the night time of the creator which also has the same time span as Brahma Kalpa, i.e. 4.32 billion solar years as per middle time units).
It is divided into four stages and in the first three stages of this Naimittika Pralaya, the lower three worlds (starting from earthly worlds) get destroyed … And in the 4th stage of this Naimittika Pralaya, the rejuvenation process of the Galaxy restarts again.
This is the stage where amongst the seven upper worlds (i.e. Bhu Loka, Bhuvar Loka and Svar or Swar Loka, Mahar Loka, Jana Loka, Tapa Loka and Satya or Brahma Loka), three lower worlds (i.e. Bhu Loka, Bhuvar Loka and Svar or Swar Loka) begin disintegrating and thus get “mostly destroyed”.
Thus, basis the above paragraph, to destroy these three lower worlds which also exist in the Milky Way Galaxy, three other Galaxies need to collide with the Milky Way Galaxy and that too in three different stages.
The fourth stage of this Naimittika Pralaya is when the rejuvenation of the Galaxy restarts.
Thus, the cyclic stages of Galactic collision which relates to our Milky Way Galaxy (i.e. the cyclic process of Milky Way collision) always take place after the completion of each of the day time of Brahma (i.e. after the completion of each Brahma Kalpa or simply, Kalpa), is what Naimittika Pralaya actually means.