Many large events are planned months in advance, and cancellations are often an issue for those attending. If the event is for a family celebration, it is unlikely that a last minute guest will be invited. For events outside of the home, arranging for a substitute is an acceptable practice. Concerts, plays and even planned trips to amusement parks are all times when people would rather inside a casual friend than waste a ticket they have already bought. It is at this time that an unexpected date can occur.
When a group of family or friends plans a major outing, they generally buy their tickets months in advance to save on expenses. Their trip might be planned down to the last detail, but they seldom plan for one member of their party to be unavailable due to work or illness. These are the times when a friend or new acquaintance might be invited as part of group, and they are often single.
Any large group of people will generally have at least a few single people in it, and inviting another single along makes little difference. If family and friends see it as an opportunity to make a match, they will do their best to be discreet. They might do small things like ensure the two sit together for a meal, or they might encourage them to go on the same rides at an amusement park. It will be casual, but the pressure will be noticed eventually.
An unexpected date does give some latitude to a couple being pushed into it, and they can always find excuses to remain away from each other within the group. If they hit it off when they first meet, the group will be likely to give them plenty of space to enjoy the day as a couple.