17

17 March is Saint Patrick's Day, observed in Ireland, Northern Ireland, and Irish diaspora communities worldwide. The feast day of Saint Patrick, patron saint of Ireland, held to mark the traditional date of his death. It is a public holiday in the Republic of Ireland and Northern Ireland, and is widely marked in countries with large Irish diaspora populations.

There are 17 years between emergences of the longest periodical cicada broods (North America).

There are 17 elementary particles in the Standard Model (the Standard Model, complete since the Higgs boson was confirmed in 2012). Twelve fermions, four gauge bosons and the Higgs. The count is of particle types: antiparticles and the colour states of quarks and gluons are not counted separately, and counting them differently is why other figures circulate.

The rest mass of a proton is about 17 × 10⁻²⁸ kilograms. 1,836 times the mass of an electron, which is most of why an atom's mass sits in its nucleus.

17 is the atomic number of chlorine. It is what makes table salt a salt, and small amounts added to drinking water and swimming pools kill waterborne pathogens — one of the largest public-health advances of the twentieth century.

A traditional Japanese haiku follows a 5-7-5 syllable (strictly, 'on') structure across three lines, totalling 17 — one of the most widely taught facts about the form outside Japan, though real haiku composition is more nuanced than the syllable count alone.

A superstition in Italian custom holds that seventeen is treated as unlucky, because the Roman numeral XVII rearranges to VIXI — Latin for 'I have lived', and so 'I am dead' (Italy). One of the few number superstitions with a specific and checkable stated origin, rather than several competing ones. Italian buildings and aircraft omit 17 where English-speaking ones omit 13. This is a scholarly account rather than the tradition's own framing.

Presented as belief or tradition, not as an established fact.

Gauss proved in 1796, aged nineteen, that a regular seventeen-sided polygon can be drawn with compass and straightedge alone — the first advance on the Greeks' list of constructible polygons in two thousand years. The result depends on 17 being a Fermat prime, one of only five known: 3, 5, 17, 257 and 65537. He asked for one to be carved on his gravestone.