From the Rebbe’s Farbrengen Shabbos Yud Kislev 5711:
In a letter, my saintly father-in-law, the previous Rebbe, explains the importance of observing the Yahrzeit of a Tzaddik with learning and Farbrengens. He tells of the Yahrzeit of the Mitteler Rebbe on Tes Kislev, when the Rebbe Rashab said a Ma’amar and then said that the learning and Farbrengen we do on a Yahrzeit is a Pidyon Nefesh we give the departed.
It is easy to assume that if it were possible to see the Mitteler Rebbe now and give him a Pidyon Nefesh, everyone would rush to do so. In addition, it is indeed possible to do that on this Yahrzeit, at this Farbrengen, as well as by studying the teachings of the Rebbe.