In my first game, the earliest day I could make a successful marriage proposal would be the 30th of Summer, when Mana reached 10 Hearts.
The earliest day that I could make a successful marriage proposal to Yue would have been the 1st day of Autumn, immediately after winning the 'Adventure Contest'. Yue remained at 9 Hearts prior to the day of the Festival. On the day of the Contest, she remained at 9 Hearts at dawn. When I spoke to her, however, her heart level increased to 10. If I had proposed to her then, however, I would have missed the Contest as the wedding would follow.
After I won the Contest by finding the Amethyst Herman had concealed on Blessia Island, I could have proposed successfully either to Yue or to Mana. (Mana's Heart Level had increased to 10 on the previous day, so I could have proposed to Mana on 30Summer.)
Technically speaking, it is not even participation in the Contest that makes the difference in heart levels but actually speaking to some one on the day of a Festival. I actually had Mana at 10 Hearts on the previous day. It is a little more dramatic, however, to link my acquisition of the title of 'Adventurer King' to the successful Proposal to Mana.
It is not by a full Heart that Heart Levels increase when you speak to a girl on the day of a Festival. This is true as well of Request Events. Very often, the girl will acquire another Heart after a Request is satisfied but sometimes the actual number of Hearts she has will remain the same. This makes it clear that each Request Event is worth a specific number of points rather than a full Heart. If the Eligible Girl is close to attainment of another Heart, fulfilling the Request will add another Heart to her Level, but if she is not close to the next level, she probably will have the same number of Hearts after the Request is satisfied. In the same way, speaking to an individual on the day of a Festival is worth a specific number of Love Points and Friend Points.
Here is the Wedding Event for Mana:
It is not the best I could have created, but in the interest of progress, I am going to move on to Yue's Wedding.
To propose to Mana, all I had to do was give her the Engagement Ring that Douglas relinquished to me in his Challenge Request. If I attempted to give it to her any earlier when she remained at 9 Hearts, she would have told me she was not ready yet for marriage. At 10 hearts, however, she accepted with alacrity. The wedding followed instantly. In fact, it followed so quickly that the official wedding photographer was unprepared!
Needless to say, I did not save the results, as I wish to be able to make a successful proposal to each of the Eligible Girls before I finally settle down with any one.