Three years ago, Ronald Reagan sent a personal $250 check to the District's Eastern High School band when it needed money to attend the Central Florida Fair Festival in Orlando.

Now Reagan can hear the band himself as the 135-member Blue and White Marching Machine struts along Pennsylvania Avenue today as part of the president's second inaugural parade, a planned two-hour procession of bands, equestrian units, floats, military and marching units, more mounted riders, choral groups, a dog sled pulled by huskies and then still more teams of horses.

It is no secret that Reagan likes to ride horses, so parade planners will show him a wide selection of them: Arabians, Percherons, Lippizaners, Clydesdales and others.

Four years ago, parade director Daniel B. Denning said, 450 horses were in Reagan's first inaugural parade. Today, the total may be 730.

''The president likes horses, so we tried to do something he'd like,'' Denning said. ''It may have gotten out of hand. It sounds like a lot of horses to me.''

In a tradition nearly as old as the Republic itself, Reagan, Vice President Bush, their wives and many dignitaries will be near the front of the parade, scheduled for 2 p.m. Then they will watch the rest of it from the reviewing stand built every four years on the Pennsylvania Avenue sidewalk in front of the White House.

Denning said today's parade is expected to feature more than 8.000 people, about 400 more than last time, and 11 floats, instead of the three last time. One float, flanked by the U.S. Olympic equestrian team, is to carry some of America's 1984 Olympic Games medal winners, while two others feature choral groups from Nevada and South Dakota. Miss America, Sharlene Wells of Utah, is to ride in a horse-drawn carriage.

More than 40 bands are expected. They include the 450-member All-American College Marching Band assembled for the parade with musicians from throughout the country.

''I think it's going to be one of the better ones in recent years,'' Denning said, ''short and sweet, but with a variety I don't think we saw in '81.''

The theme of this year's inauguration is ''We the People,'' a slogan that may best apply to the parade. Though most inaurgural events require a special invitation, some connection to the Reagan reelection campaign and a wad of cash, the parade can be watched for free. Reagan's first inaugural parade attracted 300,000 people, and a similar throng is expected today.

Most spectators will again watch as they stand along Pennsylvania Avenue. But 25,000 people have paid $12.50, $75 or $100 to sit on reserved bleacher seats.

As Reagan is driven solowly along the avenue, he may notice diffferences from his 1981 parade.

For one thing, the ride is likely to be smoother. The Pennsylvania Avenue Development Corp. (PADC), federal overseer of the massive redevelopment and refurbishing of the once-tawdry boulevard, has torn up the street in the last four years, removed trolley tracks and rebuilt the street with a 26-inch bed of crushed gravel, concrete and asphalt.

In addition, the PADC has replaced most of the concrete sidewalks between Third Street NW and 15th Street NW with stylish brick walkways, installed many new lighting fixtures, park benches, planters and trash cans and planted about 500 willow oaks.

The major change is that the ''Avenue of Presidents,'' as the sweeping boulevard was once dubbed, is again alive with people during the daytime and most nights. It was 24 years ago, during another inauguration, that John F. Kennedy voiced displeasure at the avenue's decrepit state and resolved to do something about it. But urban redevelopment anywhere requires years, especially in this most bureaucratic of towns.

In the last four years, however, the face of the avenue has changed, with private development worth $800 million complete or started. The National Threatre has been refurbished, the J.W. Marriott Hotel opened, the Old Post Office building restored, large office buildings constructed and parks and plazas sandwiched between them. More construction is under way or planned in the next few years.

Inaugural parades started with George Washington's first inauguration in 1789 in New York City. New Yorkers halted work, and thousands swarmed to the waterfront to watch the new president's inaugural barge arrive. The crowd was so large that the planned parade was delayed for more than two hours.

Thomas Jefferson was the first president inaugurated in the nation's new Capitol in Washington, in 1801. But one of the most memorable inaugurations occurred in 1829, when people flocked here to see their hero, Andrew Jackson, become president. A vas crowd walked along Pennsylvania Avenue as Jackson rode his horse from the Capitol to the White House, where he hosted an open house.

The gathering quickly turned rowdy, with hordes of people fighting for food in the East Room. Jackson escaped through a window, and riotous celebrants were lured outside whtn the White House staff moved tubs of whiskey-laced punch onto the lawn.

One of the guests at Abraham Lincoln's inaugural ball was John Wilkes Booth, who later became the nation's first presidential assassin. In 1909, a first lady joined an inaugural parade for the first time when Helen Taft accompanied her husband; eight years later, the first group of women marched in an inaugural parade, honoring Woodrow Wilson.

Seven parades have been held in snow and 10 in rain. With World War II still raging, Franklin D. Roosevelt vetoed the parade and inaugural ball at his fourth inauguration. Since then, inaugural events have been lavish and extensive, perhaps most conspicuously so in 1981.

But the parades have maintained a bit of the common touch.

As Jerry Wallace, the Reagan inaugural committee's historian, said, ''It's always been one of the key events because, unlike many of the other affairs, it's for everyone.''