Having attended three Independence Day parties a week ago, I was struck by the fact that all the other guests referred to the holiday as the Fourth of July. Why is that? Why do we say to each other: “Have a happy Fourth of July,” and not: “Have a happy Independence Day”?

We celebrate Christmas, not Dec. 25. Has anyone ever wished you: “Have a happy Dec. 25”? Or, “Have a happy Fourth Thursday in November”? No, we wish each other Merry Christmas and Happy Thanksgiving. We celebrate “Halloween,” not Oct. 31, “New Year’s Eve,” not Dec. 31, and “New Year’s Day,” not Jan. 1.

And we certainly don’t say: “Have a happy First Sunday after the Paschal full moon.” No, we say: “Have a happy Easter.”

The Paschal full moon, by the way, is supposedly the first full moon after the vernal equinox in the Northern Hemisphere. However, the Paschal full moon doesn’t necessarily occur when there’s a full moon in the sky. It can be off by a day or two — whatever’s needed to make computations work out.

The First Council of Nicaea, a council of bishops meeting in what is now Turkey back in 325 AD, settled on this loony way of calculating Easter’s date. It basically amounted to tweaking the traditional Jewish calendar.

As a result, Easter Sunday this year was April 24. Last year it fell on April 4. In 1213, it will be March 31.

Not counting 9/11, which is hardly a time of celebration, the only large observance in this country referred to as a date and not by what it memorializes is Cinco de Mayo.

Ironically, the event, which recognizes Mexican pride and heritage, is celebrated more widely in the United States than in Mexico. It is not Mexico’s independence day, which is Sept. 16.

Cinco de Mayo is Spanish for May 5.

That was the day in 1862 when Mexican forces — despite being greatly outnumbered — defeated French forces at the Battle of Puebla (right).

In this case, using the date and not a name makes sense. It would obviously be too cumbersome to wish people “a happy Celebration of Victory in the Battle of Puebla.”

And while the victory gave Mexicans a sort of David-and-Goliath sense of pride, it did not end the fighting.

France, which was then ruled by Napoleon III, was trying to establish a Latin empire, and despite their defeat in Puebla, French forces a year later defeated the Mexican army and made Emperor Maximilian the ruler of Mexico. The French, however, were expelled in 1867, and Maximilian was executed.

President George W. Bush greeting Cinco de Mayo dancers at the White House.

In Mexico, Cinco de Mayo is celebrated primarily in the region around Puebla and in a few other places. In the United States, however, President Bush in 2005 issued a proclamation urging US citizens nationwide to observe the day with ceremonies and festivities. There are now more than 150 Cinco de Mayo celebrations, which each year are held in more than 20 states, and the numbers are growing. Or so I read. In San Francisco, the event always draws large crowds.

Mexicans, I might note, refer to their independence day as Grito de Dolores (“Cry of Dolores”) or El Grito de la Independencia (“Cry of Independence”). It marks the beginning on Sept. 16, 1810, of an 11-year-long war that achieved independence from Spain.

The “cry” was made by a Catholic priest, Miguel Hidalgo, who standing outside his cathedral in the town of Dolores exhorted Mexicans to revolt against the Spanish.

As it happened, I visited Dolores (now called Dolores Hidalgo) in 2006 and was much impressed by the town’s garden-like plaza and Spanish Colonial buildings. The old cathedral still stands. What a charming place, I thought, for launching a revolution.