Here is the full list of all 55 Summer Meetings achievements.
Visited the oceanarium
Started dating Yuka
Lost a place to stay
Got the worst ending
Pushed someone away
Stuck with one girl from the start
Started meeting with Mei
Ran to someone but didn't speek
Someone confessed their love at festival!
Faced two alternatives
What did you think would happen?
Admitted your disloyalty
You thought it guaranteed a happy ending?
You've been caught
You were just curious what happens, right?
Went to a concert
Started dating Akari
After rejecting a confession, asked her on a date
Started dating Reina
Learned something
Four heroines met
You don't know what a promise is, do you?
Did you thought she'll appear again?
She never had a chance
Went to the sea
Seen an ending
Finished the game before it really started
Decided to return, but changed your mind at the last second
Parted in peace
That's not how a festival should be...
Not nice, is it?
You did not forget!
Learned the reason of breakup
Will it lead somewhere?
Started the game
Seen all 15 festival events!
You broke up in style!
Decided to break all bonds except one
Got back together with someone
Just... why?
Kissed at least four girls during a playthrough
Rejected a confession
Seen all epilogues
Things are getting weird
Activated #Help in settings
That's not good...
Lost a place to stay TWICE
A difficult situation turned a bit awkward
Someone got scared of you
Started dating Shiori
Seen Reina's epilogue
Seen Shiori's epilogue
Seen Yuka's epilogue
Seen Mei's epilogue
Seen Akari's epilogue