Responsa for Sukkah 108:4
ואין מוסיפין על מ"ח: ולא והא איכא ערב הפסח שחל להיות בשבת דאי לרבי יהודה חמשין וחדא אי לרבנן חמשין ושבע
And never more than forty-eight. No? But is there not the eve of Pesah which falls on Shabbat, on which according to R. Judah, there were fifty-one blasts, and according to the Rabbis there were fifty-seven?
Teshuvot Maharam
You asked about a community that wished to declare a fast on Monday, Thursday and Monday, and the first fast day fell on Tu Bishvat. Should they be concerned and postpone the fast to the following week or not? Thus my inclination is that the fast should be postponed to the following week, and the fast should not be set because we do not find any fast on Rosh Hashanah.
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