Reference for Rosh Hashanah 36:1
דאמר רב חנא בר ביזנא אמר ר"ש חסידא מאי דכתיב (זכריה ח, יט) כה אמר ה' צבאות צום הרביעי וצום החמישי וצום השביעי וצום העשירי יהיה לבית יהודה לששון ולשמחה קרי להו צום וקרי להו ששון ושמחה בזמן שיש שלום יהיו לששון ולשמחה אין שלום צום
seeing that R'Hanah B'Bizna has said in the name of R'Simeon the Saint: 'What is the meaning of the verse, Thus had said the Lord of Hosts: The fast of the fourth month and the fast of the fifth and the fast of the seventh and the fast of the tenth shall be to the house of Judah joy and gladness?<span class="x" onmousemove="('comment',' Zech. VIII, 19.');"><sup>1</sup></span> The prophet calls these days both days of fasting and days of joy, signifying that when there is peace they shall be for joy and gladness, but if there is not peace they shall be fast days'! - R'Papa replied: What it means is this: When there is peace they shall be for joy and gladness; if there is persecution,<span class="x" onmousemove="('comment',' Lit., 'decrees of the Government'.');"><sup>2</sup></span> they shall be fast days; if there is persecution but yet not peace, then those who desire may fast and those who desire need not fast.<span class="x" onmousemove="('comment',' Since these fasts were at the time of this Mishnah optional, no messengers were sent forth on their account.');"><sup>3</sup></span>