During the Musaf service on Rosh Hodesh, we recite a special paragraph that begins "Elokenu V'Eloke Avotenu Hadesh Alenu," in which we pray for different blessings for the coming month. As noted by the Kaf Ha'haim (work by Rabbi Yaakov Sofer, 1870-1939), this paragraphs contains twelve different requests for the new month, such as "goodness," "blessing," "happiness," "joy," "salvation," "consolation," and so on. The Kaf Ha'haim explains that these twelve requests correspond to the twelve months of the year. For this reason, he adds, during a leap year, when we add a thirteenth month to calendar, we correspondingly insert a thirteenth request into this section of the Musaf service – "U'le'chaparat Pesha" – beseeching the Almighty to grant us atonement during the coming month.

Until which point during the year is this thirteenth request added?

Rav Haim Palachi (Turkey, 1788-1869), in his work Kaf Ha'haim (34:3; not to be confused with the aforementioned work with the same name), cites three different views on this issue (listen to audio recording for precise citation). The first view maintains that one adds "U'le'chaparat Pesha" on every Rosh Hodesh throughout the leap year, until the beginning of the subsequent year. The second view, by contrast, maintains that one inserts this phrase only on Rosh Hodesh Adar Sheni, the thirteenth month added to the calendar. According to this opinion, it is only on the thirteenth month that we need to add a thirteenth request to correspond to the number of months in that year. Finally, the third view contends that one adds "U'le'chaparat Pesha" throughout the first half of the leap year, until and not including Rosh Hodesh Nissan. Since Rosh Hodesh Nissan marks the beginning of a new year in certain respects, this view maintains, we do not extend the addition of "U'le'chaparat Pesha" into and beyond the month of Nissan.

As for the final Halacha, the Kaf Ha'haim Sofer (423:18; listen to audio recording for precise citation) cites a comment of the Ben Ish Hai (work by Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Vayika (19), indicating that one should in fact add "U'le'chaparat Pesha" throughout the leap year. Even beyond the month of Adar Sheni, this thirteenth request must be added to the Musaf prayer on Rosh Hodesh during a leap year. This is also the ruling of the Eliyahu Rabba (work by Rabbi Eliyahu Shapiro of Prague, 1660-1712). The Kaf Ha'haim adds, however, that if a community has an established custom to recite "U'le'chaparat Pesha" only through Rosh Hodesh Adar Sheni, then others should not object to their practice. Otherwise, though, it is proper to insert this phrase on every Rosh Hodesh throughout the leap year. This is indeed the view one should follow.

Summary: During a leap year, one adds the phrase "U'le'chaparat Pesha" in the "Elokenu V'Eloke Avotenu" paragraph of the Musaf service on Rosh Hodesh. This phrase should be added on every Rosh Hodesh throughout a leap year, except in communities that have an established practice to add this phrase only through Rosh Hodesh Adar Sheni.