Flights at this price are available daily starting on January 9 through the end of March, scattered fall dates can also be found but are slightly more at $443 roundtrip. Take note that these fares book into Delta's Basic Economy fare bucket, meaning only carry-on items are allowed. However, if you want to check a bag; bumping up to Main Cabin is pretty reasonable at $35 one-way, which will entitle you to one checked piece of luggage and seat selection. Bringing the total to $506 roundtrip, which is still a very good deal on this route.
Icelandair is also offering very cheap flights between the two cities. However, they are even more stringent with baggage and do not provide full services like free meals, snacks, and drinks on their flights. That's why I've listing Delta as the option is a better option. If you want to save on baggage costs on future Delta flights, have a look at which credit cards waive the checked baggage fees here.
As an example, I was able to find seats departing DEN on Monday, January 21 and returning from AMS on Thursday, January 31, 2019. Other dates are widely available starting in 2019.
If you're in the market to travel somewhere else, have a look at our Denver (DEN) flight deals page to whip up some travel inspiration.
Need a room with a canal view? We've got you covered with this long list of discount deals on hotels in Amsterdam on Hotewatchdog.