Question: [Sunday, 5th Teves 5781]
Should Chabad Chassidim recite Tachanun on Hei Teves?
In general, it is customary not to say Tachanun on a day on which one celebrates a miracle that took place. However, there is no universally accepted practice of Chabad Chassidim to omit Tachanun on the fifth of Teves, and in general it is to be recited as usual. Furthermore, by the original Heiy Teves the Rebbe motioned with surprise [as seen in a video of the event] at the fact that they had omitted Tachanun by Mincha and only became satisfied after being told that there is a Chasan in the room, and thus Tachanun was omitted due to him. Thus, while perhaps there is room for an individual to personally omit Tachanun due to his great personal joy that he may be feeling [if true and internal and not a fictitious excuse to escape Tachanun] this should not be done by the Chazan, or by the general Minyan.
Sources: See regarding the general custom of omitting Tachanun on a day on which a miracle took place: Lev Chaim 150; Kaf Hachaim 218/9; Tzitz Eliezer 10/10; See regarding 18th of Elul: Hiskashrus 840; Otzer Minhagei Chabad p. 21-22; When asked in 1989 whether Tachanun is to be recited on Chaiy Elul the Rebbe responded that regarding matters dependent on feeling one is not to ask. [Otzer ibid] Obviously this refers to a true feeling of joy and not a feeling that simply comes as a result of reading this response. [ibid]