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.
About your question on whether it is “possible to search for dates when the cheapest fares apply”: SNCF offers a “best price calendar”. After you select stations and an approximate date of travel, you get a calendar with a price for each date, which is the price of the cheapest ticket still available on that date. Selecting a date shows you the train details for that day with the cheapest tickets highlighted.
The website b-europe.com offers a very similar fare calendar, which I wrote about in another answer. The SNCF calendar offers a wider range of station combinations though. For example, you can’t use the b-europe calendar for Paris—Amsterdam or Köln—Brussels, even though these are covered by direct Thalys connections. The SNCF calendar does support searches for these combinations. It may however not always show you all possible trains: the SNCF calendar shows TGV and ICE trains for Paris—Frankfurt, but it doesn’t seem to show the ICE trains that cover Köln-Brussels, only Thalys trains appear.
Thalys tickets can be bought up to three months in advance and the cheapest tickets are only offered for a limited time. In my experience, if you don’t want to travel on a very early or very late train, you’d best book exactly three months in advance. You might want to check out the information about the “1st Minutes of Thalys” fare and the “Thalys early bird fare”. For example, today (April 8th), sales started for tickets on July 8th; already some of the tickets for traveling from Paris to Amsterdam are labeled as “last remaining seats” (they mean “… at this price”):
There are “Thalys last minute fare” tickets as well, but these are more expensive than the early sales tickets and I’m not sure how likely it is you’re able to book one.