Siman 492
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א יֵשׁ נוֹהֲגִים לְהִתְעַנּוֹת שֵׁנִי וַחֲמִישִׁי וְשֵׁנִי אַחַר הַפֶּסַח, וְכֵן אַחַר חַג הַסֻכּוֹת; וּמַמְתִּינִים עַד שֶׁיַּעֲבֹר כָּל חֹדֶשׁ נִיסָן וְתִשְׁרֵי, וְאָז מִתְעַנִּים. הַגָּה: מִיָּד שֵׁנִי וַחֲמִישִׁי וְשֵׁנִי שֶׁאֶפְשָׁר לְהִתְעַנּוֹת (דִּבְרֵי עַצְמוֹ). וּבְאַשְׁכְּנַז וְצָרְפַת נָהֲגוּ לְהִתְעַנּוֹת, וְעוֹשִׂין אוֹתוֹ כְּמוֹ תַּעֲנִית צִבּוּר לִקְרוֹת וַיְחַל (טוּר).
1 The fast on Mondays and Thursdays and the Monday after the Festivals. Containing 1 paragraph: Some have the custom to fast on Mondays and Thursdays, and also on the Monday after Pesach and after Sukkot. They wait until the entire month of Nisan and Tishrei has passed, and then they fast. Rema: In places like Ashkenaz and France, it is customary to fast, and they treat it like a public fast, with the recitation of Vayechal (Torah reading).