When you think of St. Patrick's Day, you normally picture leprechauns, four leaf clovers, and the color green. You don't need to own a calendar to know the holiday is upon us because many people, Irish or not, seem to adhere to the "wear green" edict. But not everyone wears green on St. Patrick's Day; some people choose to wear the color orange instead, and the Irish flag offers a clue to that delineation.

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To understand why some people wear orange, you first have to understand why people wear green. St. Patrick's Day is a Roman Catholic holiday that was first celebrated in Ireland starting in 1631. As the name implies, the holiday celebrates St. Patrick, the patron saint of Ireland.

However, Time's Ashley Ross notes that the color associated with the holiday was actually blue until the Irish Rebellion in 1798. "[Blue] was featured both in the royal court and on ancient Irish flags. But as the British wore red, the Irish chose to wear green, and they sang the song 'The Wearing of the Green' during the rebellion, cementing the color's relevance in Irish history," Ross wrote. The change from blue to green also better reflected the country's nickname of being the Emerald Isle, as well as the clover that St. Patrick used in his teachings about Catholicism.

However, not everyone in Ireland is Roman Catholic, and the Irish flag highlights the differences. While Catholics were associated with the color green, Protestants were associated with the color orange due to William of Orange – the Protestant king of England, Scotland and Ireland who in 1690 defeated the deposed Roman Catholic King James II. Therefore, on St. Patrick's Day, Protestants protest by wearing orange instead of green.

Ironically, no one wears white; the placement of the white stripe between the green and orange stripes on the Irish flag is supposed to symbolize the peace between the Roman Catholic majority and the Protestant minority.