Madrid in a day is not possible! Unfortunately two nights and one day is all I got in the beautiful city which certainly wasn’t enough time to do everything. Madrid does have a Plaza Mayor,but the centre and busiest place is Puerta del sol, a square with streets going off in various directions all leading somewhere exciting, like the main shopping district, the cathedral and palace and the national theatre. But beware there are lots of people walking around in costumes looking to take tourists money, but can also offer you a cheesy photo. Also just ignore the prostitutes that line the streets, even in the middle of the day.