We are arriving on April 26 which is the day before Kings Day, which is a huge holiday in Amsterdam. Our hotel is near Sloterdijk station. On Kings Day do the trains and metro still run and can get us into central Amsterdam??? Am assuming the Rijksmuseum is closed too?? Thanks

Use bikes to get from Sloterdijk to Amsterdam
Rijks is closed
However, there is no telling what Koningsdag will be like. You could do the counterintuitive thing and go to Haarlem for Koningsdag. A more manageable affair, not the druk crowds that Amsterdam has but most definitely a party vibe.

Rijks is open on Kings Day according to their site.
There is no way of knowing how mad it will be in Amsterdam, but I suspect it will be busy assuming Covid restrictions aren't back in place. People will be wanting a party. If it is too busy it will possibly affect public transport.
As Menachem suggests you could head to somewhere like Haarlem instead. Not as crowded but still fun.

I visited Haarlem on Kings Day a few years ago and it was great fun. Crowds were very manageable.

Good to know that Rijks is open. Thanks for the correction.

Nice article on Expats, on Koningsdag in Haarlem. https://expatshaarlem.nl/kings-day-in-haarlem/

we are planning a trip to the Netherlands during the week of Konigsdag and because of Covid,we are wanting to be especially careful and conservative and avoid big crowds and masses of people, especially drunken people. It seems wise to avoid Amsterdam at this time, as one of the replies suggested, which we can arrange to do. My question is whether being in other cities will be similar or not? Haarlem was mentioned? Are there big celebrations in every town and city? Are most activities open or closed for the holiday? Thanks for your help with this.

There are activities in almost every town, often street markets, children's games. These are easily avoided if you don't want to go. It's a happy celebration, not masses of drunken people. You'll see lots of children and families, putting up stalls and selling stuff.
Trains to/from Amsterdam get very busy, so avoid those, and stay away from places like Leidseplein with lots of bars.
Would you worry about visiting a town in the US on the 4th of July?

Most places have something for Koningsdag (with 2 ns- koning means king) but usually much lower key than Amsterdam and other cities, often just a kids market, maybe some live music in the evening, happy people wearing orange.
Most museums and attractions are open as normal, but could be busy. Public transport to and from Amsterdam can get very crowded, and has been stopped altogether on rare occasions. This year it may be busier than normal in Amsterdam as Dutch people feel the need to celebrate after two years of it not happening, but it is generally a friendly family celebration.
If you want to avoid it in Amsterdam then perhaps head up to Enkhuizen, visit the open air museum and see what they have going on in town. Have a plan B for getting back to Amsterdam though just in case...
Or rent a car and go visit somewhere else in the country for the day.

Hoorn and Alkmaar will be nice too. But be aware that when Koningsdag in Amsterdam winds down, after about 6PM, trains out of Amsterdam will be full, often there are train disruptions if you travel towards Amsterdam as well. At least from Haarlem, there's an easy connection by bus to Amsterdam, so you can avoid trains.

Also please be aware that NL's covid policy is lax regarding civic protection. Get as good a mask as you can practically use (FFP2). There's almost no consciousness of covid's aerogenic nature.

I was there for Kings Day a few years ago. heti tried to warn me. Amsterdam was crazy. Absolutely crazy. It was like every college football teams' fans were in the city and partying. Bizarre. Really bizarre was being in the Anne Frank house and hearing the partying just outside the walls. I was having lunch a couple of blocks away, to makes sure I was not late for my tour, and was seated alone at an outside table. While I was taking photos of the sign on the window there was movement around me and a stag party from the UK joined me at my table. It was a wild adventure. They were hilarious - and bought my lunch - but the entire experience was surreal. Looking back, I wished I had followed heti's good advice to take the train to another town for the day. I did plan to visit the Museum Van Loon, and although their website said they were open, they were closed for the weekend. I was very disappointed. If you like to drink and party, you'll enjoy those around you. The entire city was orange.