What’s the Date?
There are many possible halachic prohibitions associated with counting according to the secular calendar.
Some months of the year are named for types of avodah zarah (idol worship). It is a halachic issue to mention avodah zarah by name, especially in such a respectful manner.
Likewise, enumerating the years by the “Common Era” is proscribed by many poskim.
We have a mitzvah to calculate months in accordance with the Jewish calendar, as derived from the pasuk (verse), “Hachodesh hazeh lachem rosh chadashim,” (This month [Nissan] shall be for you the head of all months). Counting according to the secular calendar--beginning with January instead of Nissan--may be a violation of this mitzvah.
We should therefore be careful to avoid referencing the date by the secular count. In circumstances where it is necessary, many have the custom of using the term “l’misparam” (according to their reckoning) as a form of disassociation.
The days of the week are also named for forms of avodah zarah. In addition, we are specifically advised to count in connection with Shabbos: Yom Rishon [B’Shabbos] (the first day until Shabbos), etc. Yet we find that historically, especially in the Yiddish language which has adopted these names, the custom has been to use the lay terms. Many poskim are lenient in this regard, since we already linked that day to Shabbos while reciting Shir Shel Yom (Song of the Day during the Shacharis prayer).