Here is the full list of all 9 Blooming Nightshade achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Spend Christmas Eve with Kiku.
Spend Christmas Eve with Atsuko.
Spend Christmas Eve with Nanami.
Spend Christmas Eve with Kiyomi.
Spend Christmas Eve with Izumi.
Witnessed two of your friends become a couple.
Spend Christmas Eve with Shiori.
You couldn't find or keep a romance alive until the end of the year...
Got through the first month of the semester.