Negative commandment · Lo Ta’aseh #151 of 613
Not to eat the second tithe while in mourning
שלא לאכול מעשר שני באנינות
The verse · Devarim 26:14
לֹא־אָכַלְתִּי בְאֹנִי מִמֶּנּוּ וְלֹא־בִעַרְתִּי מִמֶּנּוּ בְּטָמֵא וְלֹא־נָתַתִּי מִמֶּנּוּ לְמֵת שָׁמַעְתִּי בְּקוֹל ה׳ אֱלֹהָי עָשִׂיתִי כְּכֹל אֲשֶׁר צִוִּיתָנִי
“I have not eaten of it in my mourning, neither have I consumed any part of it when unclean, nor given of it for the dead: but I have hearkened to the voice of the Lord my God, and have done according to all that thou hast commanded me.”
The Rambam’s words
Not to eat the second tithe while in mourning, as [Deuteronomy, ibid.] states: "I have not eaten from it while in mourning."
שלא לאכול מעשר שני באנינות, שנאמר "לא אכלתי באוני ממנו" (דברים כו,יד).
Why · Sefer HaChinuch #608
To not eat the second tithe in bereavement
“It is from the roots of the commandment [that it is] because the consecrated foods are the table of the Higher realm, and it is not fitting for someone who is worried and very hurt in his heart to approach the table of the King.”
Who & when
“And this prohibition is practiced by males and females at the time of the [Temple], as there were tithes from Torah writ then.”
Where in the Mishneh Torah
Hilchot Second Tithes and Fourth Year's Fruit הלכות מעשר שני ונטע רבעי