This letter addresses the Siyum of Maseches Pasachim, linking the laws of Paschal sacrifices, circumcision, and redemption. The Rebbe explains how these mitzvos signify the covenant between G-dliness and the physical world, ultimately leading to the final Redemption and the blessing of Shehecheyanu.
https://www.torahrecordings.com/rebbe/igroskodesh/016/006/6093