Many French people believe that the locution « se mettre sur son 31 » means that you get well dressed for a Happy New Year - and often over-dressed -  but they ignore the origin of the phrase. It seems to go back to the Middle ages when in France « the trentain », was a luxurious cloth made of 30 X 100 threads that only rich people could afford. Another theory is that in Prussia, every 31^st of the month -  7 months out of 12 - soldiers would get a supplementary ration. Their military barracks being inspected, the soldiers needed to polish everything and appear as smart as possible.

Funnily enough in English « se mettre sur son 31 » translates into be « dressed up to the Nines » and its origins are very similar: one theory is that tailors used nine yards of material to make a suit. The more material you had the more status. And another theory is that it comes from the name of the 99th Wiltshire Regiment of the Duke of Edinburgh, known as the Nines, which was renowned for its very elegant appearance, though as the Oxford dictionary states it, the regiment's HYPERLINK "http://www.oxforddictionaries.com/definition/sartorial"sartorial reputation seems to have dated from the 1850s, while the first recorded use of the phrase is from 1837…