BOSTON — Back in the early 1970s, businessman David Mugar had an epiphany.
It happened late one night after hearing the Boston Pops play the "1812 Overture" at a Tanglewood concert in Lenox, Mass., and then watching a fireworks display nearby.
"I had this idea [along with legendary Pops conductor Arthur Fiedler] that if we combined the fireworks show with the '1812 Overture,' and if we could find some live church bells and howitzers [a short cannon], it might create a lot of excitement and bring people back to the Esplanade," he says, referring to the declining attendance at the Pops' concerts on the grassy banks of the Charles River in Boston.
Twenty-five years later, Beantown's Fourth of July celebration has grown from 50,000 attendees that first year to more than 300,000 people today (one-quarter of whom comes from outside New England), and it reaches millions of viewers on television nationwide. Attendance for this year's 25th anniversary is projected to be about 500,000.
Each year on Independence Day, spectators begin lining up, sometimes as early as 6 a.m., to secure a front-row spot along the banks of the Charles River.
Julie Downs, who has lived in Boston for 10 years, isn't an early riser. Instead, she rides her bike to beat the traffic in the early evening. "There is nothing else like [this event]," Ms. Downs says. "If you can deal with the crowds and a little traffic, you are in for the best free night of entertainment of your life."
The Fourth of July concerts on the Esplanade, first conducted by Mr. Fiedler in 1929, were nothing new when Mr. Mugar came along. But Mugar did help put Boston on the map, says Mayor Thomas M. Menino. "It's a national event now, and he's the one we have to thank for it."
"I have gone to the Esplanade concerts since I was in my late teens," Mugar says, "and I always thought they were wonderful. I set out to try to save something, and without realizing, wound up creating something."
Not only does Mugar serve as executive producer of the event, but he also picks up the tab, which runs him more than $500,000. Mugar, who is chief executive officer of Mugar Enterprises Inc. and chairman and co-owner of Blackstone Entertainment, has considered bringing in corporate sponsorship, but he is cautious. (Corporate sponsors' signs on the Esplanade for other Pops concerts are taken down for the Fourth event.)
"For me to continue to fund all of July 4 ... it's a lot. But I've always been so afraid that companies will come into the July 4 event and give away their free products, cereals ... [and have] beer signs plastered all over the place.
"Then the event itself becomes very commercial, and you forget the intent of why you're actually there. It's a pretty special day for this country, and it's a good idea to maintain that."
Money is one thing. Planning is another. Just two weeks after the Fourth, Mugar and his team begin planning for the following year. Materials must be ordered; music and themes must be formulated.
Ken Clark, who is the artistic director of Boston's Pyrotechnology Inc., has designed the fireworks display since 1983. He says that in the early days, fireworks were electrically fired with manual switching. "The first year  was a modest affair compared to today. It has grown so complex that there would be no way to do it without computers.
"But it's still a hometown display for us," he says. "We're really not interested in firing the most shells. We're trying to make an artistic statement."
After almost a year of planning, Clark and his team begin setting up the equipment one week beforehand.
On five huge barges, a crew of 20 combines 80 tons of sand, thousands of mortars (short-barreled cannons), and about 35 miles of wiring. When the setup is completed, more than 7,500 pyrotechnic devices are set in place and connected.
For the second year in a row, the 30-minute fireworks show will be synchronized to music, also known as a pyromusical. Some people don't realize, however, that some of the fireworks are done to pretaped Pops music. "The only time we collaborate live with the fireworks is when we do the '1812 Overture' and 'The Stars and Stripes Forever,' says Boston Pops conductor Keith Lockhart.
This year's special guests
The program this year will feature the Broadway cast of "Ragtime," the Creole band Buckwheat Zydeco, and pop singer Melissa Manchester in a George Gershwin tribute. Lockhart, who will lift his baton for the fourth time as Pops conductor for the Fourth, says that "it's quite a rush to perform for such a large audience."
"When people think of this country and its origins, people think of Boston, Philadelphia - the cradles of liberty. It's a place that people want to be on the Fourth of July," he says. "The same way people want to be in Times Square on New Year's Eve."
* 'Pop Goes the Fourth!' airs live tomorrow, 7:30-10:30 p.m. EDT, on A&E.
Lisa Leigh Parney's e-mail address is email@example.com