Everybody will agree that the Netherlands is an interesting country to visit.
But that weather ! You never know what to expect !
You can indeed encounter three seasons in one day over here.
Now, there is good news and bad news for visitors to Holland.
Yes, the weather is pretty unstable, with on many days some risk of precipitation.
But on the other hand, this changeability also means that, if it rains, it generally does not rain for a days on end.
So, either you can resent the climate, and grumble. Or you can simply take this fact into account in your planning.
By including flexibility ! That really is the key word for this country.
And being flexible is the more possible with tools like rain radar on the internet. You can make an accurate short term forecast for your day.
"Well, it's going to rain a lot this morning, let's visit that and that museum, cosy indoors."
"Hey, looks this afternoon the weather will be OK. Let's make a bicycle trip" (for example).
True, the best period of the year for outdoor activity is April till October. But even in winter a short bicycle trip of a couple of hours can be just great. Especially with a good meal at the end of the day. How about cooking yourself a traditional Dutch or historic dinner ?
About days of the week to do things. Many museums are closed on Monday. Sundays tend to be very quiet in small towns, with shops, local museums, and restaurants often closed. Even in towns like Amsterdam expect live to really wake up around noon on Sundays.