Reference for Taanit 2:9
אֵין גּוֹזְרִין תַּעֲנִית עַל הַצִּבּוּר בַּתְּחִלָּה בַּחֲמִישִׁי, שֶׁלֹּא לְהַפְקִיעַ הַשְּׁעָרִים, אֶלָּא שָׁלשׁ תַּעֲנִיּוֹת הָרִאשׁוֹנוֹת שֵׁנִי וַחֲמִישִׁי וְשֵׁנִי, וְשָׁלשׁ שְׁנִיּוֹת חֲמִישִׁי שֵׁנִי וַחֲמִישִׁי. רַבִּי יוֹסֵי אוֹמֵר, כְּשֵׁם שֶׁאֵין הָרִאשׁוֹנוֹת בַּחֲמִישִׁי, כָּךְ לֹא שְׁנִיּוֹת וְלֹא אַחֲרוֹנוֹת:
Public fasts are not to be ordered to commence on a Thursday, in order not to raise the price of victuals in the markets; but the first fasts are to be on Monday, Thursday, and [the following] Monday; but the second three fasts may follow on Thursday, Monday, and [the following] Thursday. Rabbi Yose says, "Even as the first fasts are not to be commenced on Thursday, so also are the second and last fasts not to commence on that day."