Talmud for Taanit 2:8
כֹּל הַכָּתוּב בִּמְגִלַּת תַּעֲנִית דְּלֹא לְמִסְפַּד, לְפָנָיו אָסוּר, לְאַחֲרָיו מֻתָּר. רַבִּי יוֹסֵי אוֹמֵר, לְפָנָיו וּלְאַחֲרָיו אָסוּר. דְּלֹא לְהִתְעַנָּאָה בְהוֹן, לְפָנָיו וּלְאַחֲרָיו מֻתָּר. רַבִּי יוֹסֵי אוֹמֵר, לְפָנָיו אָסוּר, לְאַחֲרָיו מֻתָּר:
Wherever it is mentioned in "The Roll of Fasts," that "no lamentation and mourning is to be made" on certain days, it is also prohibited to do so on the day preceding, but allowed on the day following them; but Rabbi Yose says, "It is prohibited to do so on the day preceding, and on the day following; where it is said, that 'no fasts are to be kept thereon,' it is allowed to fast on the day preceding and following days." Rabbi Yose says, "It is prohibited on the preceding, but allowed on the following day."