1) There is a debate regarding the best way to identify the secular months when writing invitations, documents etc.. Harav Ovadia Yosef zt”l (Yabia Omer 3 YD 9) cites the Ramban who stresses the importance of counting months according to the Jewish calendar. When the Torah says that Nissan is the first month of the year, it is implying that one may not consider any other month to be the “first.” Therefore, Harav Ovadia concludes, when writing the secular months one should not refer to them by number, but by name. For example, one should write January, 12, 2016 and not 1/12/2016.25 A similar ruling is expressed by Harav Shlomo Zalman Aurbach zt”l. (Shalmei Simcha page 687)
2) The Tzitz Eliezer (8:8) disagrees with the ruling of Harav Ovadia Yosef zt”l. He writes that because the months are named after gentile gods, one may not mention the months by name, but should instead refer to them by number.
3) Harav Hershel Schachter shlit”a, Rosh Yeshiva of Yeshivas Rabbeinu Yitzchak Elchonon, in a letter addressed to the author, writes that he feels that it is preferable to write the name of the month than to reference it by number. Harav Noach Isaac Oelbaum shlit”a, in a letter addressed to the author, explains that it is difficult to advance a clear hallachic ruling. However, he personally adheres to the view of Harav Ovadia Yosef zt”l and refers to the secular months by name and not by number.