The English version of the expression seems to be mudflat hiking, as you can see on this wikipedia page about it
In Dutch it is called 'wadlopen' and when you google that you will find many locations, wadloopcentrum Frieslân is a site that combines several locations. On the Wikipedia page above you will find link to more sites, I use the Wadlopen Frieslân site for the rest of the answer as it has a lot of information in a handy format.
The explanation text is translated into English, the menu part at the bottom is not, the menu at the top is in English and leads to English pages.
The bottom menu lists: 'Wadlopen' (clothing and shoes, FAQ, conditions, photographic impression) 'tours' (list of locations, use a map to find them) 'dates' (list of months May to October inclusive) 'links' 'about us'(with our guides) and 'contact' (with phone number)
Each of those is a link.
In an other site I found that they do tours round the year, but those are only close to the island Texel and return to base, so the length of the tour can be adjusted to the weather.
As they explain in the text, if you want to do a tour and it is not available, they will try to find you an alternative. In the height of summer there will be more tours, early and late in the season it will be down to the weekends and maybe one extra in a school holiday week.
In the 'mostly' English page for tours you find a description, followed by a classification. 1 to 5 out of 5, with 5 being the hardest, followed by length in km, time in hours and price in Euro.
On the dates page you can make reservations but if you want a tour from them, you can also just phone or e-mail, their FAQ page mentions that the reservation page is sometimes hard to work.
Wadlopen is walking over the sand when the sea is out in the shallow areas. But usually there are stretches which are mud and cannels of water you have to wade through. The time is restricted due to the tide and the distances are quite big.
You need to go with a guide and a good guide as well as there have been disasters and needed rescues are quite common. As in a couple of times each year, even with the best guides. The good ones all have a way to contact a boat to come out and get you to safety.
If you are serious about trying it, send a mail to one of the companies offering tours, all businesses in the Netherlands will be able to communicate in English, certainly by mail. When you call, speak slow and clear, start with a couple of sentences that allow for yes/no answers or common niceties, so they can get into speaking English before having to think about difficult things.
On the page shoes and clothing you will see a photo of people in canvas trainers, those are the kind of shoes most appropriate. Buy cheap ones and throw them out when done, they will be very dirty. Shorts but warm tops and extra layers, as it will almost always be cold but long trousers will just get wet and make you misserable. And needed is a rain and wind proof jacket. Bring extra clean clothing to change into when done with the walking.
Wadlopen as photographed by GerardM on 5 March 2004 see here for more information
Some of the tours are returning to the starting point, others go out to one of the islands and the return will be by ferry, or you can walk from the island to the main land in some cases.
You should ask whether to bring food, as it does depend on the time of the tour, but the ones I checked out did not provide food and they mention that you are expected to want to eat more than usual, as it is hard and cold work.
I have never done any 'wadlopen' but I have heard from people who have done it that it is an incredible experience. Hard, wet, cold, but something you will not forget lightly.