The US fast/abstinence rules are very confusing and full of change due to 1) the indults granted during WWII which halted/minimized many penances, 2) the post-WWII re-introduction of fast/abstience by Pius XII, and then 3) the 1962 and 1969 changes which re-halted/deleted the vigils/penances all over again. To say that the US (and Europe's) fast/abstinence laws were "inconsistant" during the 40s-70s is an epic understatement.
Lastly, I read where the fast/abstinence rules were switched from the Assumption to the Immaculate Conception, for the US only (due to our specific consecration to the Immaculate Conception).
Traditionally, (i.e. in normal catholic times, pre-1900s), I read where there were over 70 vigil days kept by Christianity (not including the 40 days of Lent). I've heard a Byzantine priest say that they still fast/abstain "almost 1/3 of the year", which is consistant with the above numbers.https://acatholiclife.blogspot.com/2014/08/vigil-of-assumption-of-blessed-virgin.html
The Catholic Encyclopedia around the time of St. Pius X in the early 1900s mentions: "In the United States only four of theses vigils are fast days: the vigils of Christmas, Pentecost, the Assumption, and All Saints." The Vigil of the Assumption however ceased being a fast day by the early 1950s. **This is probably due to the switch to the Immaculate Conception**
However, its observance as a fast day is ancient as the Catholic Encyclopedia states, "Pope Nicholas I (d. 867), in his answer to the Bulgarians, speaks of the fast on the eves of Christmas and of the Assumption...The Synod of Seligenstadt (1022) mentions vigils on the eves of Christmas, Epiphany, the feast of the Apostles, the Assumption of Mary, St. Laurence, and All Saints, besides the fast of two weeks before the Nativity of St. John."