Train from Rouen to Paris on Sunday May 4th

If trains sold out months in advance they would be useless. The French wouldn't have it. If you are trying to book far in the future and seeing trains as not available that means that they just have not opened for purchase yet. The 90 days you sometimes see mentioned is not a hard deadline. Different railways do different things, and commuter trains operated on behalf of the regions are yet another thing...

If you look on sncf-connect you will get a notice that there are negineering works planned, which will affect the schedule, and that trains will soon be available...

Download the SNCF app. It's very user friendly. It's can send you notifications when the schedule is available; and it'll keep your tickets/QR code in the app and easily accessible for when you board.

We did find routes were more limited on Sunday's in general.

Currently the main line from st. Lazare to Rouen is often closed on Sundays because of the works to extend the RER to Mantes. The trains then get rerouted via Argenteuil which adds about 10 minutes. But SNCF often publishes these schedules not until closer to the date. So yes, wait a bit.

I have read that there will be extensive disruptions from St Lazare train station to Normandy area this spring. Here are the dates

• from Saturday 15 to Sunday 16 February
• from Saturday 29 to Sunday 30 March
• from Saturday 5 to Sunday 6 April
• from Saturday 12 to Sunday 13 April
• from Saturday 26 to Sunday 27 April
• from Thursday 1st to Sunday 4th May
• From Thursday 8 to Sunday 11 May
• From Saturday 14 to Sunday 15 June
• from Saturday 8 to Sunday 9 November

I don’t speak enough French to determine if there are alternatives. But check SNCF schedule for a Tuesday after your date of travel and if they show available then it appears it is the train works that make booking a train impossible.

I bought a train ticket from Rouen to St. Lazare Sunday April 6 last week.

The trains to Vernon/Giverny Monet’s gardens from St Lazare are the ones that I was most interested in. In fact the website for Monet’s Gardens mentions the train disruption and to check before you book tickets. So maybe that line is more impacted than others to Normandy.

I bought tickets for the Garden for Easter 2023 a couple of months ahead. Trains OK. Before I left for my trip I recheck all my trains, Giverny OK. Holy Thursday I checked again. Train going to G-V OK, but no train back from there to Paris. Went to the station, called the train company, bizarrely no train. I called the Garden and they said I could use another time. Since I was going to be in Paris again November 2023, I went then. Since then always make sure I can get someplace and more importantly get back.

Trains for the Monday, Tuesday, Wed, etc after 5/4 are available. But no trains on Sunday. From everyone's comments, it appears there may be track work and no trains will be available on Sunday 5/4. Is there any way to confirm, or best to change our travel plans and avoid Rouen that weekend?

Flixbus is showing frequent buses from Rouen to Paris on May 4.

This is Europe. Major railway lines are simply not closed with no substitution. If it is not possible to run any rail service by a diversionary route then bus replacements will be provided.
You are simply being impatient and working to the kind of very long advance schedules which Europeans simply don't work to.

If you look on 13 April you will see that there is a train from Paris to Rouen every two hours with the note-

Attention
Info Travaux Trains Nomad-Des travaux sont prévus ce week-end sur votre ligne pouvant impacter la fréquence et le temps de parcours. Les trains proposés tiennent compte des travaux.

That translates as -
Careful- Nomad train works info- Work is planned this weekend on your line that may impact the frequency and travel time. The trains proposed take into account the works.

Likewise two weeks later.

Currently for 4 May the advisory reads-

Des travaux sont prévus du 1er au 4 mai sur votre ligne impactant fortement la fréquence et le temps de parcours. Les trains seront disponibles prochainement. Pour votre confort nous vous invitons à réserver votre voyage en dehors de ces dates.

OR

Work is planned from 1 to 4 May on your line, which has a strong impact on the frequency and travel time. Trains will be available
soon. For your convenience, we invite you to book your trip outside of these dates.

IE- There will be train service, but schedules are yet to be confirmed. This is nearly 3 months away yet.