Having organised Dates with almost every Eligible Girl for Autumn, reloading afterwards in each case, I have discovered a few useful rules about Dates in general:
1. You can wait until the day prior to the actual set Date to ask any Girl for a Date, but on the actual Date day set by the game, you will not obtain the Date option from ANY Eligible Girl. In other words, your 'last chance' to ask any Girl for a Date is the day prior to the Date;
2. There are two different Date Events: If the Eligible Girl is at 8-9 Hearts when you meet her for a Date, you will experience the 'short form' Event that will end BEFORE the picnic and you therefore will not have the option to propose to the Girl. If, however, the Eligible Girl is at 10 Hearts, she will bring food for a picnic and, after consuming the food noisily, you will have the option to propose to her. Whether she accepts or refuses depends on whether you have met all Marriage Requirements, both general and specific. In other words, you need to have completed the Girl's Story as well as the Double Bed and so on.
3. If you make a Date with one Girl and then ask a second Girl for a Date, you will lose almost a full Heart with the second Girl but the Girl you attempted to 'two-time' will remain oblivious of your dishonourable conduct. Your original Date will stand and her heart level will remain constant.
4. If you make a Date with an Eligible Girl and then 'stand her up', there are NO consequences whatsoever. Her Heart Level remains constant and she will not even scold you for not appearing!
5. If you tell the Girl after eating her food that it was NOT delicious, there will be no negative consequences in terms of Heart Level. In fact, in most cases, althouggh she will burst into tears, she will tell you rather docilely that she will try better in future!
6. Whether you meet the Girl 'early' or 'late' is irrelevant in terms of changing her Heart Level. There usually is a different dialogue depending on the time when you meet her. For a noon Date, you can meet the Girl ordinarily as early as 11.30 a.m. and as late as 1.00 p.m. but she will leave the screen shortly after 1.00 p.m.
As a matter of fact, if you are NOT at Lake Poli at 11.30 a.m., the Eligible Girl will walk into the screen at approximately 11.40 a.m. but if you enter the screen at 11.30 a.m., you will find her there and be able to initiate the Date Event immediately.
7. A very minor point, but one that I tested. Once you initiate the Date Event, you must see it through to the end, whether or not it is the 'short' or 'long' form of the Event. After walking to the centre of the bridge for the Autumn Date, you cannot leave without experiencing the next part of the Event. If you attempt to run away, the Girl will stop you again and again until you speak to her voluntarily to share your enjoyment of the view!
I am making a video about all of this... I have footage of poor Selphy standing at the entrance to the bridge humming the Song of Trust while my character stands there without engaging her in dialogue. After 1.00 p.m., she sadly leaves the area...