Although the book of Judith was never considered part of the Hebrew Bible, it is undoubtedly of Jewish authorship and, as Tal Ilan writes, it provides a window into ancient Jewish religious practices:
The book of Judith was composed sometime after the Hebrew Bible was completed. It came into being, however, considerably earlier than the Mishnah and the Talmud. Thus, Jewish customs recorded in Judith were influenced by the Hebrew Bible and reflect an earlier Judaism than that practiced today. The Jewish customs in Judith relate to fasting, widowhood, kosher food, immersion, conversion, and slavery. . . .
While at the Assyrian camp, Judith prepares and eats her own food, refusing table-fellowship with the Assyrian general Holofernes. This custom is part of the Jewish dietary laws of kashrut. . . . One might [conclude] from this custom that table fellowship with foreigners on their own “turf” was [also] prohibited.
Also while in the Assyrian camp, Judith goes nightly to the nearby spring to immerse herself. Immersion was practiced in Second Temple Judaism to remove impurity. It was also practiced by sectarians such as the Essenes on a daily basis, as a sign of piety. Immersion in Judaism today is practiced only by women after menstruation and certainly not on a daily basis, but Judith’s daily immersion is a sign of her piety.