n0mesorah,
Additionally, you seem to be מדייק in the word “Rejoicing of Rashbi” in the Rebbe’s letter.
This was not the Rebbe’s words (it’s a translation). The Rebbe wrote “הילולא דרשב”י” – which he always uses to describe passing. The yahrzeits of the Chabad rabbeim are often referred to as יום הילולא.
This is taken from the term which the Zohar uses about the passing of Rashbi: ואתכנשו להילולא דרבי שמעון (Zohar Idrah Zutah 296b).