I have an illustration of that precise point as a few years ago my dh and I were asked to our dil's 'big' birthday at our son's house. We were told that we would be going out at some point, so I assumed it would be for a country walk as they don't do big parties, and wore a plain winter dress and boots and my big coat. I did have my hair done. I knew that her parents would be there and possibly her brother and family. I made some salads as my contribution and picked up a little bouquet, a bottle of champagne and a balloon to give our dil. Her mum made a birthday cake. Her brother had illness in the family, so it was just two sets of parents who arrived. We were all having a chat when they suddenly said they had an announcement to make. I was overjoyed at the thought of their engagement and then our son said ' we're getting married '- pause-' at two o'clock today!'
My dh had to scrape me up off the floor, but we absolutely had to go along with it. They had decided against a big wedding, and although in many respects it was strange, it was wonderful as well. A colleague of mine said later that she had two sons, and that if they did that she would 'kill' them. But it was their day, their decisions. My only regret is that we didn't get very good photos as we were still in shock, but even that didn't disturb our son and dil.