A fascinating case study in non-rationalist Judaism occurred yesterday. Several sources, including religious news outlets and the rabbinate of Modiin, sent out an announcement that it is dangerous and forbidden to drink water. Specifically, to drink it yesterday afternoon, between the hours of 2pm and 3pm.
This caught a lot of people by surprise. The reason given was even more surprising: that this hour is the transition point of the summer solstice, which, like the winter solstice and the equinoxes, divides between the four "quarters" of the year. As such, it is a time when the angels change shifts, and while there is nobody on duty, the Angel of Death can poison the water.
Before you rush to dismiss this out of hand, you should also be aware that while this is not mentioned in the Gemara, it is mentioned by no less an authority than the Rama (Yoreh De'ah 116:5), who notes that this is a "basic" custom that is a tradition from many great authorities.
So what are we to make of this?
The original reasons for this practice are lost in antiquity (see some discussion and references at this link). The given reasons appear to be based upon a view of Judaism and the universe that is, shall we say, not shared by rationalist Jews today. And so some would say to reject it as a pagan superstition.
But on the other hand, it is codified in halachic sources. This should give it a certain degree of authority, even if the original reason for the practice is without basis. (Cf. killing lice on Shabbos.)
Yet on the third hand, this is a practice that pretty much nobody has cared about or known about in hundreds of years. In light of that, accompanied by its non-rationalist basis, I would say that it is a similar case to the prohibition against eating allegedly lethal peeled onions and eggs that have been left overnight. It's a tradition that has no rational basis, no value, and has quietly died. There's no benefit to bringing it back from the dead.
Of course, if it does end up getting a full resurrection, as seems to be happening with the peeled onions, it's going to be difficult to justify ignoring it.
I would like to add another point. Several years ago, I gave a lecture about rationalist Judaism, lamenting how the Orthodox community has gone steadily in the non-rationalist direction over the last few centuries. Dr. Marc Shapiro, who was in attendance, disagreed - he pointed out that nobody is terrified of demons today, as they used to be. I think that this is certainly the case here. People who are resurrecting the prohibition against drinking water during the solstice are not actually afraid that it is dangerous. (Note that due to the calendars falling out of sync, yesterday was not in fact the solstice!) These people just want to do what they think makes them frum.