When did your toddler drop transition from 1 nap to no naps?

LO is 20 months. For the longest time he used to wake up at 7-8am, nap by 1pm, then go to bed at 7:15pm. He used to just pass out by 1pm, and now he just won't go down! He also used to nap for a good 3 hours straight (or more if I let him) on the weekends. That was blissful. Recently he naps 1-1.5 hours. It is freaking me out a bit because I am not ready for no naps yet!

What is/was your 20 month nap/sleep routine like? When did your toddler transition to no nap?