Anniversaries

Factual Questions
1

Anyone out there know why in the U.S., the 25th, 50th, 75th, etc. anniversaries of major events are important? (i.e., weddings, founding of companies, etc). I’ve always thought that it was tied to the length of a generation (25 years), but my father-in-law recently pointed out that in Japan, anniversaries such as the 30th have more cultural significance.

I’ve checked encyclopedias and customs books, but no luck so far.

2

just easy divisions of a century of course. The japanese don’t use (or didn’t) use the western calendar, so maybe 100 years is less meaningful.