The Luach Colel Chabad is a calendar published annually by Colel Chabad. It includes instructions for special prayers and Torah readings according to the Chabad custom. It also mentions red-letter days for Chabad, whether they affect the prayer service or no. Among these are the yahrzeits (anniversaries of death) of the Lubavitch chief rabbis and a few other rabbis important to Chabad. But not all rabbis or great people — not even arguably the greatest — are included. The yahrzeits of Moshe, Aharon, and the Ari, for example, are not mentioned. But the yahrzeit (Tamuz 15th) of Rabbi Chayim ben Atar, the Or Hachayim, is mentioned in the calendar (PDF). Why is it?
See the story about the Baal Shem Tov knowing about the Ohr HaChayim's death because he (the Baal Shem Tov) had received the secret of hand washing, which had previously been entrusted to the Ohr HaChayim.
Chassidic tradition is that the main reason the Baal Shem Tov twice tried so hard (and failed) to get to the Holy Land was that he said if he could join the Ohr HaChaim there, together they could bring Moshiach. He is buried outside the walls of the Old City of Jerusalem.
See also the story here, titled: 'Journey to the Holy "Ohr Hachaim"'
I have no idea if it's relevant, but the story goes that the Noda Bihuda's daughter was, unbeknownst to her father, a fan of the teachings of the Baal Shem Tov (the "0th Lubavitcher Rebbe.") While talking with him, at some point he abruptly tore his garments. Weeks later they received the news of the Or HaChaim's death -- at the date and time the Baal Shem Tov had suddenly torn his clothing in mourning! She told her father this was proof that Baal Shem Tov had divinely-inspired knowledge.
See the entry for Ohr Hachayim in Shem Hagedolim
אור החיים - פירוש התורה להרב המופלא חסידא קדישא מ' חיים ן' עטר ונדפס בוויניציא סביב לחומש עם תרגום ורש"י. ושמענו כי עתה בפולוניא הם מחשיבים אותו הרבה ונדפס עוד שני פעמים ונתעורר זה על ידי שהרב החסיד קדוש מהרר"י בעל שם טוב הגיד גדולת נשמת מהר"ח הנזכר
My translation: Ohr Hachaim: Commentary to the Torah by the outstanding Rav and holy Chassid our master Chaim Ben Attar, published in Venice surrounding the Chumash with Targum and Rashi. We heard that now in Poland they esteem it highly and it was published another two times. This was inspired because the Rav and holy Chassid our master Yisrael Ba'al Shem Tov told of the greatness of the aforementioned Rabbi Chaim.
Considering that the Ba'al Shem Tov is the founder of Chassidus, and teacher of the Maggid of Mezritch, who taught the Ba'al HaTanya, who founded Lubavitch, it makes sense that they would make a note of his yartzheit.