I often see huge announcements for incredible fares for Thalys trips between Paris, Brussels, Cologne, and Amsterdam. Whenever I need to use the Thalys, I am never able to get these incredible fares. You always need to provide dates when you want to travel. Is it possible to search for dates when the cheapest fares apply?
This blog post on the Capitaine Train site explains that to get the cheapest fares on Thalys and many other European carriers, you need to book well in advance. In the case of Thalys, their booking window opens 90 days before the date of travel, so most of the cheapest fares will be snapped up early in that window.
If you only find a need to travel, say, a couple of weeks ahead of time, you may have already missed out on the best fares. On the other hand, if you know 90 days or more ahead, then the start of the booking period is the sweet spot.