I had an almost identical experience over 50 years ago. A foggy Christmas Eve in my backyard in Santa Clara armed only with a flashlight, a screwdriver and a pair of pliers. Thankfully, it was a pretty minimal set, just a couple of swings and I got it together before dawn. A few years later, having gotten a much larger set (swings, glider, slide, rings etc.) and having learned my lesson, I did some pre-assembly and "hid' the components in the rafters of the garage. Subsequently, my pre-school age son announced that he knew what Santa was bringing for Christmas. When I asked how he knew, he took me into the garage, pointed to the rafters and said "see the swing set up there"!