A bit of Torah knowledge would help, especially for Mashiach Agent. YK can never occur on friday because of lo adu rosh. that obviously makes Tzom gedalia never able to fall on friday. because the calendar from pesach to tishrei is fixed, tisha bav and shiva aser be’tammuz never fall on friday. because 2 of the months between tishrei and adar, can have 29 or 30 days, asarah betevet can occur on friday and we fast. when to end the fast once shabbat starts is a machloket at least currently. a personal taanit or a community taanit like erev parshat chukat can happen on friday.
we do not realize that in the communities of ashkenaz (then largely germany and northern france) ALMOST ALL copies of the talmud and other manuscripts were destroyed and there was no printing until 300 years later.