The September 4th issue of Jinja Shinpō carried an article about the revival, after 150 years, of a matsuri at Hirota Jinja, in Hyōgo Prefecture. The matsuri in question is centred on the public display of a jewel: a crystal with a flaw in it that looks like a sword. According to the jinja’s account, this crystal is the “wish-fulfilling jewel” that Okinagatarashihimë (also known as Jingū Kōgō) is said, in the Nihonshoki, to have found before travelling to conquer the Korean peninsula. As Okinagatarashihimë is also said to have founded Hirota Jinja on her return to Japan, it is easy to explain how the jinja came into possession of this treasure. The sword inside the jewel is said to always point west, and the legend says that this was an omen telling her to conquer the peninsula.
There are a lot of reasons why this cannot be true (“Okinagatarashihimë did not actually exist” is the most decisive), but it seems to have been very important at the jinja, and was incorporated in a Nō play in the fourteenth or fifteenth century. In the Edo period, the jewel was shown publicly every year on the seventh day of the seventh month, but this custom was abandoned in 1871, shortly after the Meiji Revolution.
The article does not say anything about why the practice was abandoned, and it is not immediately obvious. After all, it is a legend closely associated with the military success of an Imperial ancestor, which is exactly the sort of thing that the Meiji government was interested in promoting. However, the practice of keeping an item hidden most of the time, but showing it to the public on special occasions, was, and is, closely associated with Buddhism. In Shinto, things tend to be kept hidden all the time. Thus, I suspect that this matsuri was abandoned because the whole concept felt too Buddhist.
The revival is held on the first Sunday of August, because that tends to be close to the seventh day of the seventh month on the old calendar, and is also a day when most people are not working. From the description, it sounds fairly simple: a standard matsuri, performed in front of the jewel. The article specifies that about 240 people came to see the jewel, so it is hardly a mass phenomenon — yet.
This sort of revival of matsuri that have been in abeyance for decades, and sometimes even longer, is not that uncommon, although I do get the impression that matsuri falling into disuse because of depopulation is more common.