52

There are about 52 weeks in a year. A common year is 52 weeks and one day, which is why a given date moves forward one weekday each year.

A standard deck of playing cards has 52 cards, split into four 13-card suits — close to, though not derived from, the 52 weeks in a year.

The 52-card pack is a French standardisation from the late fifteenth century: four suits of thirteen ranks, with the simple pip shapes — hearts, diamonds, clubs, spades — that could be printed from stencils rather than hand-painted. Earlier European packs varied in both suit count and card count, and regional decks that never adopted the French pattern still do: the Spanish baraja has 40 or 48 cards, the German pack 32 or 36.

The ISO 8601 week date numbers weeks 01 to 52 — or 53, because 52 weeks account for only 364 days and the leftover day or two accumulates. A year gets a 53rd week when it starts on a Thursday, or on a Wednesday in a leap year, which happens roughly every fifth or sixth year. Payroll, retail reporting and scheduling systems built on 52-week years have to handle that extra week explicitly.

The French-suited pack of fifty-two is standard in English-speaking countries, while Spanish, Italian and German traditions use forty or thirty-two card packs. The fifty-two card deck's global familiarity follows distribution rather than any inherent advantage.

In the history of Maya and other Mesoamerican cultures, the two calendars realign every fifty-two years, completing a Calendar Round. The 260-day ritual count and the 365-day solar year return to the same pairing after 18,980 days. Among the Mexica the completion was marked by the New Fire ceremony, with household fires extinguished and relit. This is a scholarly account rather than the tradition's own framing.

52 is the atomic number of tellurium. Paired with cadmium it forms the thin-film compound behind one of the two main commercial solar-panel technologies.