Recent studies have shown that people are more generous and happier when the sun is shining. And now I just read about a study done on twitter that people are happiest in the mornings. In the afternoon they slump and are more likely to feel sad and drepressed. By the evening they rebound somewhat. This holds true even on the weekends.
So it seems to me that if you want to convince someone to do something pick a sunny day and in the morning. Seems like an odd time to ask for a date...but what the heck!