140:2 On the Shabbos before Purim, we read parshas Zachor (Deut. 25:17-19). On the Shabbos before Rosh Chodesh Nisan, we read parshas HaChodesh (Exodus 12:1-20). If Rosh Chodesh Nisan falls on Shabbos then that day is parshas HaChodesh and we act as we do when Rosh Chodesh Adar falls on Shabbos. (See above, 140:1.) On the Shabbos before parshas HaChodesh, we read parshas Parah (Numbers 19:1-22).
140:3 Some authorities say that reading parshas Zachor and parshas Parah are Biblical requirements, therefore we may not call a minor for maftir on those weeks. Similarly, people in villages that do not have a minyan must go to a place where there is a minyan for these readings. If this is not possible, they should at least read these portions according the proper tunes.