Where and when is the best time to see the tulip fields in the Netherlands?

Annoyed already answered most of your questions (he recalled correctly) but I wanted to add some extra info.

A nice bonus to the flower season is the annual "bloemencorso" this is a parade, much like a typical carnival parade in other parts of europe, but with the cars all done up in flowers. This year it is held on the the 3rd of may, so at the end of flower season. It is not optimal to combine with the flowers in the field (because those field flowers are used to make the cars) but if you come a week or so early you can combine the two.

If you are in the Netherlands, and want to enjoy the tulips "the dutch way" then there is no better way than by bike of course! Follow the "bollenroute". Start at Leiden train station, rent a bike there and off you go! At the far point you reach the keukenhof so you can combine the ride with a visit there. It is a 37 km ride, which may or may not suit your fitness level, but if the weather is nice this is absolutely the best way to enjoy the tulip fields! Two things to keep in mind before you decide that biking is not for you:

• The Netherlands is flat as a pancake. The highest point you reach on this route is probably 5 or 10 meters above sea level (on a high bridge somewhere), and the lowest is probably 2 or 3 meters below.
• You should check this, but there is a good chance that they rent out e-bikes at the train station.

For the best view on the fields, take a tour.
As the tour drivers check on the fields and keep each other updated, you have the best chance of seeing the best views of the fields.

Most fields are 'harvested' as soon as all flowers are open, as the flowers are cut off to improve the growth of the bulbs to get a better flower next year.
That means that most fields are only in flower for a few days, so it is hard to get a good view with many fields in flower by random choice or by accident.

There are flower fields along most of the western coast of the Netherlands north of the Hague, and there is a train line running through most of that area, but not all fields are convenient close to the railway line.

The spring flowers bloom, depending on the kind and the weather, from February to June, with April as most likely to see a good selection of fields.
In the rest of the year you might see other flowers. In the same areas there are growers who develop new variations or who just propagate for selling the plants, and for cut flowers. In some cases the flower stems will be cut before the flowers open until there are too many coming to the market and the cost of harvesting is bigger than the money they sell for. This makes for unexpected fields full of flowers. So any time you are traveling in the Netherlands you should keep your eyes open, as you might see 'fields full of flowers'.