Today is Valentine's Day. Since anybody reading this likely already knows the day's traditions, I'll focus on the history of the holiday.
From what I can find, the exact origins of Valentine's Day are lost to history. However, it likely comes from the ancient Roman feast of Lupercalia in February. Lupercal began with a sacrifice of male goat(s) and a dog along with spreading the blood from the sacrifice onto 2 nude Luperci. (Male goats were symbols of sexuality.) Then, the actual feast. After the feast, the Luperci cut strips from the hide of the sacrificed goat, ran around nude (or nearly nude), whipping women with the strips. At some point, men drew the names of local women to find out who they'd be coupled to for the duration of the festival, sometimes longer. Over time, Lupercalia's nudity was replaced with everybody being fully clothed.
In the 3rd century CE (AD to us older folks), there's a tradition stating that Emperor Claudius II executed a man named Valentine on 14 February after Valentine tried to get Claudius to convert to Christianity. There's another legend stating that Claudius had 2 men named Valentine executed on the 14 February, but in different years. Despite the lack of records confirming the legend, Valentine was made a saint by the Catholic church. I couldn't find a date in my quick search though. However, Pope Gelasius outlawed Lupercalia and declared 14 February Valentine's Day in the 5th century CE.
Valentine's Day was romanticized by both Chaucer and Shakespeare in their writings. During the middle ages, the tradition of handmade cards became popular. During the Industrial Revolution, factory made cards were first created and gained popularity.
To finish this off, here's a map with the countries that celebrate Valentine's Day in pink.