That explains it. I was wondering why the sudden flood of Rumia.
What about Mystia? She seems to arrive in waves, too.
Hmm, funny since Mystia doesn't actually have a well known date...
Maybe the day for Unagi is overlapping...?
Then again I checked and there were actually not much for that tag...
Dates on pixiv:
6th: Reimu day (06 = "Rei" "Mu")
April 6th: Youmu day ("4/06" "Yo" "Mu")
8th: day for those who missed Rumia day
9th: Day of Akyuu or Cirno... the again "9"th month is Cirno too...
April10th is day of Jealousy, A.K.A day for Parsee. (Jealousy in Japanease is Shitto →Shi & tto → 4 &10 = April 10th)
14th and 15th are days for Tenshi. (14=10+4 "Ten" "Shi" 15=10+5 "Ten" "Ko" )
16th is day for Sakuya (Izayoi=16th)
19th is day for Iku
Is that it? Dunno.
Some other stuff that may be hitting are changing the character's names to numbers or relating words...
Like 890016 being Reimu and 1341398 being Sakuya, 378 can be Sanae... etc.
... Also, spring is always the season for her, what ever the day may be.