- I don't understand the "instead of reproducing" bit. The names for months and weekdays in Lojbo culture will surely be named by lujvo. But the categories of months and weekdays in European culture (e.g. the fact that there are 12 and 7 of them, and that they are associated with certain gods, emperors and history) are foreign concepts and therefore better named by fu'ivla than lujvo. As for why native Lojban elements are used, there is no single European language, so it is unclear which language should be borrowed from, in some cases (e.g. Monday). I suppose that "December" could be rendered as mastrdekimu, drawing from Latin, but mastrdau is a calque-like borrowing that has its own kind of legitimacy (borrowing meaning rather than form).
Lojbo culture no doubt employs some scientifical nomenclature for divisions of time, but ropno culture has a common nomenclature deeply rooted in its history. Here are possible fuhivla for the European categories.