D-Day plus 20 plus 20! The 40th anniversary of the Allied landing at Normandy is being observed by CBS with a newly edited version of a classic 1964 documentary, ''D-Day Plus 20 Years.'' The original featured a long meandering walk through the beachhead area by Gen. Dwight D. Eisenhower and Walter Cronkite.
This time around, CBS Reports: D-Day and Eisenhower (Tuesday, June 5, 8-9 p.m.) is being shown in reedited form with a new introduction by Cronkite. It remains a fascinating though painful piece of historical nostalgia.
''D-Day'' starts in the war room at Southwick House where the invasion was planned, and it recalls step by step the execution of the bold mission that resulted in the liberation of Western Europe. The general recalls his upsetting emotions as he sent his men off to possible death.
Illustrated with newsreel footage from the actual invasion, as well as footage of a youthful Walter and Ike as they wander in the bootsteps of the British and American troops 20 years later, the documentary spares viewers nothing - the uncertainties, the terrible risks, the cemeteries where the heavy casualties lie.
Instead of the customary broadcast credits at the end of this extraordinary film, viewers will see an ''order of battle'' listing every major unit of the Allied Expeditionary Force which was actively engaged in the assault on the Normandy beaches on June 6, 1944.
The film is a commemoration rather than a celebration. And it is appropriate that war-weary Eisenhower ended his participation on a solemn note: ''I devoutly hope that we will never again have to see such scenes as these.''
Executive producer of the original film was Fred W. Friendly; in the current version the executive producer is Burton Benjamin. (Meanwhile, NBC-TV has announced that on June 6 from 8 to 9 p.m. EST, it will be airing ''D-Day Plus 40 Years,'' anchored by Tom Brokaw, with live coverage of the commemorative ceremonies in Normandy.)
A phone chat with Walter Cronkite
Twenty years after he made the original 20-years-after documentary, Walter Cronkite is preparing to fly to Southwick to film a new introduction. But he takes a few minutes out of his busy schedule (he is anchoring two new ''CBS Reports'' - one on Harry Truman's 100th birthday and the other on the high-tech revolution) to talk about how reporters covered World War II.
''Things were more in the open in those days,'' he says. ''We knew all about the invasion and we were keyed up for it. Although there was complete armed-forces control of the press, we went everywhere. They would never have thought of mounting a military operation without the press present.''
Not at all like the way the press was handled at Grenada last year?
He laughs. ''The attitude was 180 degrees in a different direction from Grenada.
''There was a feeling (during WWII) that the American public was entitled to know what troops were doing in combat, entitled to get a full, impartial history of what went on. Whether or not people could be told immediately because of military security was not important. What was important was that impartial observers be present. ''The military welcomed that presence. There was an attitude that the military were the servants of the people, that this was the people's operation and the people were entitled to know.''
Mr. Cronkite says he believes the Grenada press situation was a one-time affair. ''I think that the military now appreciates that the handling of the press in Grenada was a mistake,'' and is anxious to correct it.
What was Walter Cronkite doing 40 years ago at the time of the invasion of Normandy?
''I was flying over the beach in a B-17 trying to bomb behind the beaches, but the weather was too bad for us to bomb anything. I wasn't supposed to be there - I was supposed to be in the United Press office helping to write the lead story for the day. But when the Air Corps agreed to send in the Flying Fortresses, they wanted newsmen to go along to record the history of the event. I was the guy whose name was picked out of a hat.''
Cronkite says that the most unexpected aspect of his interview with Eisenhower was the discovery that Ike was such an emotional man who felt deeply about the responsibility of sending so many young men into battle. And he had ''a great deal more knowledge about the details of the operation than I had previously thought.''
Cronkite is delighted that CBS has decided to rerun the D-Day show, because so many young people seem to have forgotten - if they ever knew - the details of that historic day. ''And, you know,'' Walter Cronkite says pointedly, ''I believe like the philosopher Santayana that those who cannot remember the past are condemned to repeat it.''