Dear Rabbi, When we say Birkhat Hachodesh for a two day rosh chodesh, I was always under the impression we say both days using the following formula, ’ראשׁ חדֶשׁ פלוני יִהְיֶה בְּיום פלוני וּבְּיום פלוני’, In my shul this shabbos (Rosh Chodesh Kislev, Thursday and Friday), the chazan added ’ולמחרתו’ between the days. I am aware this formula should be used for Rosh Chodesh which is on Shabbos followed by Sunday, but was wondering if this is correct for a two day rosh chodesh which does not start on shabbos?
ב"ה Shalom Questions of Minhag=custom are always interesting. The foremost answer to this type of question is what determines what is correct is the Minahg of the Shul and what the Rabbi of the congregation has to say. Nonetheless, I will go into a short background of the custom of announcing Rosh Chodesh. Rashi in his Machzor Vitri (סי' קצ) mentions that when Rosh Chodesh is two days we say the original day and add on ויום פלוני"" with no differentiation between two days in the same week or Shabbat followed by Sunday. Approximately 300 years ago, in the Sefer נוהג כצאן יוסף (עמ' קמו) ,which recorded the customs of German Jewry, it says that if Rosh Chodesh is on Shabbat followed by Sunday,the custom is to say Rosh Chodesh is on Shabbat and ולמחרתו ביום הראשון . This is also mentioned in other books mentioning the customs of German Jewry. What I have seen as an explanation for this addition of "…ולמחרתו" is to prevent the distant possibility of one confusing the Sunday following the Shabbat of Rosh Chodesh with the Sunday immediately following Shabbat when we bless the new moon. Even so, it seems that among the Chassidim there are those who add " ולמחרתו" only when Sunday follows Shabbat Rosh Chodesh, (Custom of Belze Chassidim) and in the Sefer דרכי חיים ושלום , of the Rebbe of Munkatsh zt"l , it says that his custom was to add " ולמחרתו"whenever Rosh Chodesh was two days. Also, in the sefer שבט הקהתי או"ח קפב:, it says the addition of " ולמחרתו" when Rosh Chodesh is two days can be even in the middle of the week. So to sum up. Though, it may very well be that the original custom was to add " ולמחרתו" only when Rosh Chodesh is on Shabbat followed by Sunday, from our short survey, we have seen that somewhere along the line, the custom diversified . So as I began, the custom to be followed in your Shul is whatever the time honored custom is. All the Best and Chodesh Tov