Train from Paris to Bayeux

Go to the official taxi stand at CDG and take a taxi from CDG to Paris St-Lazare Train Station.
If you can buy your ticket on a mobile device while in the air knowing that the flight is going to arrive on time, give yourself at least 4 hours between the scheduled arrival of the plane and the departure of your train to Bayeux

Check schedules and book tickets on the SNCF website. If you want to book straight from the airport, your origin will be Paris Roissy Charles DeGaulle Airport.

Look carefully at the details of the various routings.

I cannot provide any guidance about how much time you need to allow between your scheduled arrival time and the train departure time from the airport station. It's possible to get hung up for a considerable period at Immigration if a bunch of planes arrive at the same time and yours is the last of the group.

To clarify, there are no direct tains from Charles de Gaulle airport to Bayeux, Bayeux is on the opposite side of Paris.
Bayeux trains leave from St Lazare station (Gare St Lazare). As a first timer, you best option is to get a taxi from the airport to St Lazare. This will be a fixed-price, €50.
It is not when you arrive at the airport that counts, it is when you emerge after going through immigration, which could take 30 minutes to 2 hours. You cannot safely book in flight, as you don't know how long immigration will take.
I think you will have to take the expensive option of buying tickets when you get to St Lazare.

The easiest is to go by taxi from CDG to Gare St Lazare in Paris. It is also the most expensive. Only under certain specific circumstances would I decide to pay for a taxi from CDG to Paris. I only take public transportation, but if you're sure that arrival in CDG is at night, then choose the taxi option.

I would take a taxi. We did this a few years ago. We wound up having about a four hour break at the train station because I wanted to make sure we had enough time. We did & it was fine. Enjoy

What do you plan to do in Bayeux? Because if you're planning on renting a car then you probably don't want Bayeux as your final destination as car rental facilities are extremely limited. Instead, make your destination Caen (just before Bayeux). There are lots of car rental facilities there, and it will be an easy/short drive from there onto Bayeux.

Better yet,
Ick up a rental car at CDG. Drive as far as Vernon/ Giverny, spend tne night and continue to Bayeux stopping at Honfleur en route.

Most likely I will be able to book while we are in flight.

Probably not a great idea. In flight you will at best know what time your plane arrives at CDG. You won't know when you'll be at the train station on the far side of Paris as others have pointed out.

Let me suggest 2 options:

1) buy the train tickets when you arrive at the train station. The automated machines are easy to use. There are trains every 2 hours or so (you can look them up online at the DB site https://www.bahn.com/en/view/index.shtml I wouldn't want to this if I was arriving at CDG in the evening though.

2) spend your arrival day in Paris. If you arrive in the morning you have time to walk around, visit a museum or two, maybe even do a Paris Walks tour. If you arrive late in the afternoon or evening then it might be just dinner. But since you sound confident you will arrive sometime on May 7th this takes the stress and wasted time out of the equation. Whenever you arrive at CDG you go to your hotel (I concur with taking the taxi). The next morning you take the train out to Bayeux.

Well I went on the oui sncf website and purchased our return tickets from Bayeux to Gare St. Lazare. We are talking about flying out that Sunday and staying near the airport (???) and then taking the train to Gare St. Lazare in the morning to head to Bayeux. That was we would be able to get our tickets ahead of time. Renting a care is out of the question for us. We don't have any desire to do that. Thanks for all the great replies!

Note that you will not be taking an SNCF (the agency that takes you to Bayeux) long-distance train from CDG to St. Lazare. One likely route is the RER subway from CDG to a change at La Chapelle, and a second RER train to Gare St. Lazare. I have experienced big crowds for tickets at CDG. I bought a torn-apart Carnet ticket from a tout, working the line of tourists who were still out of sight of the turnstiles! It turned out to work fine.

Stay in downtown Paris near to St Lazare...don't waste your time at the airport hotels.