This type of logic doesn't work for every episode Nana makes an impact, but a good amount are related to the number 7 in some way.
>episode 2 and 9
2 is the first episode she appears and 9 is the 7 mystery episodes. 9-2=7
>episode 25
Definitely an impactful Nana episode, the one where she redeems Zakkuri. 2+5 = 7. She also had significant screentime in episodes 22 and 24, which take place in July.
>episode 29 and 30
9-2 = 7 and 0-3 = -7. The time has come for Nana to fix Meroron... maybe.