Anyone interested in travelling on Sept 8/9 or 15/16 must wait till mid-August before they can purchase tickets. See below for the latest announcement from Thalys.com:
"Due to planned engineering works on the High Speed Rail Lines in France, booking tickets for the following dates is not yet possible: 8 & 9 September and 15 & 16 September 2018.
The reservation shall be back online as soon as we receive confirmation about the impact on our traffic and the final transport plan is made.
The re-opening of the ticket sales for these dates is expected by mid-august.
Please accept our apology for the inconvenience.
Message updated on 2018-07-18 at 16:46:40."