30th day of month

There is a halachic debate about how to handle this situation.  Possibilities include: not observing yom hachodesh, since there's no day thirty; observing the twenty-ninth, since the woman became niddah at the end of the month; observing the first of the following month, since the woman became niddah on Rosh Chodesh. 

If you have a rabbi that you usually turn to, we suggest that you ask him what opinion to follow. If not, our position is that you should observe the first day of the month this coming cycle (1 Sivan).  Since the thirty-day interval comes out on the twenty-ninth of Iyar, you will, in effect, satisfy all the positions.  Should you in fact become niddah on the first of the month, there would be a possibility of establishing a veset kavua for Rosh Chodesh.

Please write back with any further questions.