Rashi in Meseches Yevomos68 states that the yartzeit of a tzaddik is a regel, and some poksim say based on this, no tachnun is recited on this day.69
68. 122a “Tulsa.” Refer to Meshmeres Shalom 12:4, Avodas Ephraim 2:pages 510-515 on why some have the custom not to recite tachnun on a yartzeit of a tzaddik. Some say a yartzeit is like a chasuna (ibid). See Kaf Ha’chaim 580:24 who brings that the day of a yartzeit of a tzaddik is a day to fast not a regel. Refer to Shem Aryeh O.C. 14.
69. Meshmeres Shalom ibid, Ketzos Hashulchan 24:badi 19.