Former president Jimmy Carter sent his regrets, explaining that he would be salmon fishing on the Matapedia River in Quebec, with an ABC-TV crew filming him for a future episode of "The American Sportsman."

But several thousand other Democrats have decided that, all things considered, they'd rather be in Philadelphia.

From Friday through next Sunday, they will be gathered en masse for the first time since their August, 1980, Madison Square Garden convention ended on a sour and downbeat note presaging the November defeat.

Party chairman Charles T. Manatt, who is staging the affair, hopes the atmosphere in the City of Brotherly Love will be a lot more fraternal than it was in New York. To that end, he has changed everything about the meeting--including its name.

In the vernacular of the party, this is a mini-convention--the third mid-term gathering in a series that began in Kansas City in 1974 and continued in Memphis in 1978.

But Manatt insists that it be referred to in all publicity as a national party conference, not a mini-convention. The 897 voting delegates are not to be called delegates; they are participants. Alternates are not alternates, but replacements.

The intention is to erase any resemblance, not just to the family brawl in the Garden, but to the divisive mini-conventions of the past.

In 1974, Democratic governors had to mediate a fight over the new party charter, and when they caved in to demands from blacks and women activists, the leaders of organized labor walked out.

In 1978, the mini-convention became a showcase for Sen. Edward M. Kennedy's first full-scale attack on the domestic policies of the Carter administration. Vice President Walter F. Mondale and half the White House staff spent an anxious weekend, rounding up votes to defeat a critical resolution backed by United Auto Workers president Douglas A. Fraser and other unhappy liberals.

Nobody wanted to go through another gathering like that, especially the Democratic members of Congress who are looking ahead to the fall campaign. Many urged Manatt to cancel the mini-convention.

When he convinced them that it had been mandated by a resolution passed in New York, they obtained a promise that he would keep it from becoming a playground for activists passing far-out resolutions that the Republicans would happily trumpet as official Democratic policy.

He has done his best to be sure nothing unprogrammed happens in Philadelphia.

Instead of encouraging states to conduct grass-roots elections of delegates, as in the past, Manatt arranged for almost half the delegate seats--374--to be filled by members of the Democratic National Committee. Another 320 delegates were chosen by state committees, often from their own membership. Another 100 or so slots were given to elected officials, and Manatt himself picked the last 100 delegates.

The rules he rammed through the national committee effectively prevent any policy debates from coming to the convention floor. On Saturday, the delegates will participate in seven issues panels. Draft statements will be open to debate and amendment there, and any dissent that gets a quarter of the votes in the room will be included in the panel's report.

At next Sunday's final session, unless the rules are changed, the only vote permited will be a vote to "accept" the panel reports.

For the most part, the draft statements put together by Manatt's staff echo the 1980 platform and indict "failures" of the Reagan administration's domestic and foreign policies. The one notable new area is a statement "welcoming" the nuclear freeze movement and endorsing a negotiated, mutual, verifiable freeze of U.S. and Soviet strategic weapons.

Despite all that has been done to remove drama and surprise from the Philadelphia agenda, it is turning out to be a big event.

More than 1,000 journalists have applied for credentials and estimates of total attendance range from 5,000 to 7,000 people.

The big draw for many is the parade of 1984 presidential hopefuls, who will be speaking during the weekend and running hospitality suites. At Friday afternoon's opening session, Mondale and Sens. Alan Cranston of California, Ernest F. Hollings of South Carolina, Gary Hart of Colorado and John Glenn of Ohio will orate--along with the official keynoter, House Speaker Thomas P. (Tip) O'Neill Jr. of Massachusetts.

Arranging the batting order for the White House aspirants has been a task of delicate diplomacy for Manatt, but his job was eased late last week when Kennedy agreed to move out of the Friday lineup and become the featured speaker on a long program on Sunday.

That was a great relief to the others, who, remembering Kennedy's speechifying triumphs in Memphis and Madison Square Garden, were understandably reluctant to appear back-to-back with him.

In addition to the speeches and issues panels, the mini-convention will offer dozens of workshops on campaign techniques and an array of fund-raisers for the party and various constituent groups.

Manatt said he also hoped it would set out "some of the basic themes" of the fall campaign, and special presentations have been arranged on issues like Social Security, economic "fairness" and women's rights, where the Democrats believe Reagan and the Republicans are vulnerable.