septem

Septem is indeclinable, as all the Latin cardinals are between quattuor and centum. The number never takes an ending, so the noun carries the case on its own: septem dierum, "of seven days", septem militibus, "to seven soldiers". Learners who try septorum are inventing grammar.

The ordinal is septimus, the distributive septeni, "seven each", and the adverb septiens, "seven times". September was the seventh month of a calendar that began in March, which is why it is now the ninth and nobody has ever fixed the naming. Seven does a lot of symbolic work in this corpus - the seven sacraments, the seven gifts, the seven hills - so it turns up far more often than its arithmetic would predict. French sept, Spanish siete and Italian sette all descend from it.