Any festivals or things to do in Amsterdam on April 20 2019?

There are indeed lots of 420 events going on in Amsterdam that day.
There are lots of smaller events at the various coffee shops, but one of the major events occurs outside city hall (Waterlooplein tram & metro station) during the day; free of charge. There is also a more elaborate events in the outskirts of the city featuring cannabis merchants and musical acts.
More details here:
https://www.420fest.nl

You would probably be surprised at how many people know about 420.

I'm sure there will be things to do in Amsterdam on 04/20. I've never been there on that day and I don't participate in the associated activities. Unfortunately, if you have never been to Amsterdam you will probably be disappointed. While smoking is tolerated, it is not a wild in the streets anything goes place. You can be arrested if you don't follow the rules.

While smoking is tolerated, it is not a wild in the streets anything
goes place.

On 4/20 it will be. That's why one of the main events is held at city hall. It's part political rally to show politicians who there constituents are.

Surely in the Netherlands - and most of the world - the day comes first, then the month, so it should be 20/4?