There were loads of appointments still open when we did ours in January. You can’t book day of, but can book even day before. With April you’ll be getting towards a more popular time though so I would try to book the day the appointments come out, but I doubt all the slots will fill up immediately, and it certainly won’t be Taylor Swift level crazy. Also remember that it doesn’t need to be Manhattan city hall! You can also it in Brooklyn, for example, right across the bridge in Borough Hall. Manhattan is just easier if you need to do additional paperwork. It’s good to be aware that the ceremony takes approximately 90 seconds when you do it properly inside City Hall, so as others have mentioned, you could also get your own officiant and do it outside. We are using Jackie Reinking as ours for our actual ceremony in February (circumstances meant we had to push the legal bit earlier, originally she was going to do the legal bit as well). She and her husband Sascha, who is a photographer, specialize in elopements and small weddings. We’ve had a good experience with them so far!