My dictionary gives two translations for "week": septimāna and hebdomas (gen. hebdomadis, feminine). Is there a difference between these two words? Are there contexts where only one of them is valid?
The word septimana is more common in my experience and it looks like an original Latin word, whereas hebdomas looks like a Greek loan. Using a Greek loan instead of a more Latin word does already affect the tone, but I wonder if there are more nuance differences between these two words.
A look at the corpus suggests that both words were in use at the same time. Varro uses both; the latest (classical) writer to use septimana is Tacitus, the latest to use hebdomas Aulus Gellius. Hebdomas occurs thirteen times to septimana's six, but half of those uses of hebdomas are in Aulus Gellius (many of them referring to Varro, but my quick look wasn't enough to tell me which of the two Varro used in the work Gellius is referring to).
For what it's worth, Cicero uses hebdomas once but he doesn't use septimana.
However, as @fdb points out (thanks for the correction!), classical authors who use the word septimana are not using it to mean "week"—it's only Christian Latin authors who give it that meaning.
The early Romans did not have a seven-day week. The earliest evidence for a seven-day planetary week (seven days each associated with one of the seven planets) is not until the 2nd century BC. A look at the references in Lewis/Short shows that neither the Latin word septimana, nor the Greek loan word hebdomas is ever used by any classical author to mean “week”. This usage begins with Christian authors.
Here is a summary of a recent academic colloquium on the origin of the week: http://blogs.ucl.ac.uk/calendars-ancient-medieval-project/2015/07/08/the-origins-of-the-seven-day-week/
