Once a year, for twenty-three years now, the mile-long stretch of big-name museums along Fifth Avenue open up their doors to the public for one evening, free of charge — they call it the Museum Mile Festival. Fifth Avenue itself closes to automobiles and becomes a promenade, and the street floods with art lovers. I sound like I know it all, but I’ve only just come home from my first Museum Mile Festival after repeatedly missing them year after year. I got to see the Frank O. Gehry retrospective at the Guggenheim, which was great because it’s great and also great because it was free. My only complaint is that the festival, which runs from 06-09p, is too short — by the time I was done with Gehry, there was barely enough time to quickly peruse the “Aluminum by Design” show at the Cooper-Hewitt.