I've gotten enough that I can fill in the blanks entirely, because the only ones I'm missing are in "sets".
Gonna list how to get the ones I remember just for future reference.
First row: Opening Aden, Opening Sonja, Prologue - A Flash of Light, Birth of Earth Giant, A Sealing Released (Opening videos, just wait at the title screen. Prologue...start the game. BoEG, have Ymir grow. A Sealing Released happens when you beat the Wind Temple.)
Second row: Battle, Crash, Epilogue - In the Shooting Stars, Wedding Spring, Wedding Summer (Battle, Crash, and Epilogue all happen pretty quick before the end of the game. Wedding Spring and Summer...well, get married. In Spring or Summer.)
Third row: Wedding Fall, Wedding Winter, Led by Dragon, On a Giant (Daytime), On a Giant (Nighttime) (Wedding Fall/Winter, see Spring/Summer. Led By Dragon, make a wish at the Dragon Shrine for the dragon to take you to another place. On a Giant Day/Night I'm not sure of, Night is one of the four I'm missing, as well as three Weddings.)