Is celebrating the 75th a Jewish thing?
There is a particular appeal to round numbers, usually associated with birthdays and anniversaries, when a big deal is made of the celebrant and their milestone. There is not much support for this common practice within Torah literature of yore. The Arugas Habosem writes that celebrating a landmark birthday may actually be chukas hagoyim (non-Jewish practice that is Biblically banned).
Nonetheless, we do find that there were Gedolei Yisroel who have commemorated these milestones publically—such as a sixtieth or seventieth birthday (often marking it as a celebration of note with the recital of shehechiyanu). It has become increasingly popular in the most strictly observant communities to mark the yahrtzeit (anniversary of passing) of a distinguished personality in a special manner when it reaches round numbers: 25, 50, 75, and on. Indeed, these events are commendable, as they enhance yiras Shamayim (fear of Heaven) among the participants. Poskim subsequent to the Arugas Habosem argue that such a Torah-true experience would have been encouraged by the author himself, and his admonition applies to the type of festivity that has negative results.
When the guest of honor is an adam chashuv (a prestigious individual) and a role model, or the dear departed is a tzaddik whose works and accomplishments are promoted at the commemoration, it is certainly endorsed by halachah.
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