Amsterdam to Paris on Sun 25th Aug or Mon 26th Aug not sure which is the better day. Departure during the day. Not sure how long the journey is. Not sure which station in PAris the train arrives at.
Direct high-speed
Thalys trains depart every two hours from Amsterdam Centraal and arrive to Paris Nord (Gare du Nord) station. Monday afternoon usually is cheaper if compared to Sunday afternoon, but if you ok purchasing your tickets in advance, then it doesn't really matter, since discounted tickets are sold for all connections. Travel time with
Thalys trains are a bit more than 3 hours (3:16 to 3:25 depending on connection)
Discounted tickets are sold up to 3 month in advance, for buying it you can either use
thalys.com or Dutch railways website - nshispeed.nl/en
Discounted tickets start from 35 euro, but remember that those are tickets with some limitation (non flexible), so read tariff rules before buying
Paris to Lourdes
Here direct high-speed trains (TGV) depart from Paris Montparnasse station, while overnight train from Paris Austerlitz. Travel time for TGV trains is roughly 6 hours, overnight train is 8 hours. Once again works the same principle - the earlier you purchase your tickets the better chances to catch cheap discounted ones (fare can be as low as 25 euro, tgv-europe.com), although sometimes there is also additional promo campaigns, which might start and end time to time, so even if you didn't catch discounted tickets at first try, there is no point to buy in advance full fare tickets