Today is St. Patrick's Day, the holiday when Irish communities in the diaspora around the world (but mostly in America) dye everything green, wear clothing adorned with shamrocks, and give out pinches to people who don't dress up for the celebration. If you question the actual origins of the holiday and how they relate to modern celebrations, you're not alone; it's one of several holidays in the modern calendar that have departed significantly from their historical origins. What was once a holiday about fasting and snakes is now apparently mostly about parades and yelling about your one-sixteenth Irish ancestry to anyone who will listen. Not that there's anything wrong with that — it sounds like a pretty good time. But it's quite different from the holiday's roots.
Holidays and how they evolve are a genuine area of historical interest, but it's actually rare for them to depart significantly from their original focus. A lot of historical argument is taken up with how different holidays in various eras relate to one another (whether Easter is related to celebrations of the Germanic goddess Oestre, for instance, and how modern Christmas may have taken various traditional cues from medieval Twelfth Night revelries) — many of which are shaped by the fact that the Christian church often seemed to overtake or adapt pagan traditions among converted communities. In strong religious contexts, preservation of holiday traditions is often likely to be pretty constant; in Judaism, for instance, the replication of their calendar of traditions down the generations was a direct challenge to attempts to wipe the religion out. Various cultures, meanwhile, have simply added more elements over the course of history. This is the case with Chinese New Year, which can be traced back 3800 years to agrarian festivals around the beginning of spring, and gradually accumulated various now-traditional elements from early exploding-bamboo fireworks to eating yusheng, raw fish salad.
So the concept of a holiday veering quietly sideways is actually quite a rare thing — but as not as rare as you might think.
St. Patrick's Day: Originally About Bacon & Snakes
Like another saint's day, Valentine's Day, St. Patrick's Day was originally centered on a significant day in the life of a Christian saint. Saint's days were hugely important in the Christian calendar for much of the last millennium, with one for every day of the year (and sometimes several); people who were named for certain saints would celebrate a "name day" on the day devoted to the saint in question, a tradition that continues to this day. In the case of St. Patrick, though, things have gone in a decidedly different direction from their initial ideas.
The feast day of St. Patrick, established by the church in 1631, was devoted to the fact that he was the patron saint of Ireland, a fifth-century saint who allegedly escaped from pirates and went on to be a Christian priest who drove all the snakes out of the country. (No, there are in fact no snakes at all in Ireland.)
In celebration of Patrick, medieval Irish Roman Catholics were allowed a day off from Lent, the period of meat-free abstention and devotion before Easter, in which they were allowed to eat bacon, dance and generally carouse. The day after, though, they were expected to return to modest self-denial. It became an all-out celebration on a massive scale largely because of Irish immigration to America; the first St. Patrick's Day parade was actually held in New York City in 1762, and it's always been celebrated most strongly by the Irish diaspora across the USA.
Mother's Day: Originally A Day For Servants To Visit Their Churches
The celebration of mothers that comes from Mother's Day is partially a modern invention, but its origins actually stem from earlier Christian interpretations of a holiday known as Laetare Sunday. The mass of Laetare Sunday still includes the phrase "Rejoice, O Jerusalem: and come together all you that love her ... and be filled from the breasts of your consolation.” They're not talking about your own biological mother, though; it's about the "mother church," and was meant to be a day to go back to it. Your "mother church" was the one at the center of your community or where you'd been baptized, and you were expected to journey cross-country to get there if necessary.
Laetare Sunday, or Mothering Sunday, fell out of favor during the Reformation of the Christian church, but two women decided to bring it back in the early 20th century: Constance Smith and Anna Jarvis. On opposite sides of the Atlantic, both successfully argued for it to come back into fashion, but with an unexpected addition they didn't entirely anticipate. Celebration of May 9, which was the date on which Smith's mother had died, was adapted to the fact that, at the time, many young people worked in domestic service far from their homes. On May 9, they went back, not only to their "mother church," but to their actual mothers. The connotation of Mothering Sunday was about religion and servants' rights well into the 20th century, and only after domestic servants and Christianity ceased to dominate the cultural landscape did it get today's celebration-of-motherhood label.
April Fool's Day: Originally About Calendars & Fish (Maybe)
The origins of April Fool's Day are widely debated. There are strong traditions of fool-festivals in Roman times, for instance, from the Saturnalia (a Christmas forerunner in December that involved revelry and raucous pranks) to the Hilaria, the last day of the festival of Cybele on which masquerades and imitations of prominent people were allowed. Tightly controlled and hierarchical societies have often had periods of misrule in which everybody felt free to play ridiculous jokes and release their excess energy; this was, for instance, where modern Carnivals came from.
There's an enduring tale about the origins of April Fool's Day, though, that sets it apart from other explanations for riotous festivals. According to some historians, All Fools' Day, as it was originally called, may have dated back to 1582, in which France switched from the Julian calendar set by the Romans to the Gregorian one. (The switch took a long time and caused such chaos that correspondents in different countries would often have to write two dates on their letters, one from each calendar.) The change meant that October 4, 1582 was followed by October 15, 1582, which was understandably confusing, but it also meant that the new year was now to be celebrated on January 1 rather than at the end of March. If people didn't celebrate on the "right" date, they were apparently known as poissons d'Avril and had paper fish pinned to their cloaks. It may simply be a modern incarnation of old humor festivals, but the calendar story is much more fun.
Groundhog Day: Originally About Childbirth & Badgers
This is an extremely odd one. The idea of Groundhog Day, in which a rodent apparently predicts the weather, sounds like your typical bizarre American folklore-based idea — but it has actually got an intensely long history that relates back to Christian ideas of charity, Celtic divination, and badgers. No, really.
The idea of divining the weather during winter from the actions of ground-dwelling mammals appears to date back to the ancient Celtic festival of Imbolc, which celebrated the midpoint between the winter and spring equinoxes, and was full of bonfires, midnight dances — and acts of prediction using badgers and serpents, who lived in the cold ground. As an agricultural festival, this makes sense, and draws a pretty clear parallel to today's festivities. It's when things enter Christian tradition that they get weird.
Imbolc gradually, it seems, became associated with the Christian festival of Candlemas, in which priests blessed and handed out candles for the winter to their flock. The candles were apparently a means of predicting the length and harshness of the winter, but the origin of the festival wasn't really about the festival of light. In Christian tradition, Candlemas marked the point after Jesus's birth where Mary, who'd been deemed "unclean" because she'd given birth, was cleansed and allowed back into the church (it's also known as the Feast Of The Purification). The holiday came to America in the form of German settlers, who had a Candlemas tradition relating to hedgehogs as weather-predictors; but it's come a long way from its history of badgers and unclean women.