The Midrash Shocher Tov on Tehillim states regarding the verse (Tehillim 91) “From the pestilence that walks in darkness”: “The Sages explain that this refers to a demon (a harmful being created on Erev Shabbat at the time the world was created and is neither fully spiritual like angels nor fully physical like human beings). Rav Huna said in the name of Rabbi Yose that the ‘Ketev Meriri’ demon is made of many peels, many hairs, and many eyes. It sees with one eye and this eye is within its heart. It has no power either in the sun or in the shade; rather, its power is between the sun and the shade. It rolls around like a ball and has power from the fourth hour of the day until the ninth. It has power from the Seventeenth of Tammuz until the Ninth of Av. Whoever sees it falls over and dies.” Similarly the Midrash Rabba on the verse “All of her enemies overtook her between the straits” explains that this refers to the three weeks between the Seventeenth of Tammuz and the Ninth of Av when “Ketev Meriri” is present. This means that our Sages, who were fluent in all of the secrets of the world, tell us that there is a harmful demon called “Ketev Meriri” present during this period of the “Three Weeks” and we must therefore conform our behavior and be aware of this fact during this period of time.
One must take care not to walk around alone during these three weeks beginning from the fourth hour of the day until the end of the ninth hour, which is a total of six hours. (The way to calculate these hours is from dawn, meaning that one must begin being careful from after three hours after dawn have passed for a duration of six hours. These hours are seasonal halachic hours, i.e. in Israel each halachic hour during this time of year is approximately and hour and a third. Thus, six halachic hours amount to approximately eight regular hours from approximately 8:15 am until 2:00 pm.)
The Aruch Ha’Shulchan points out that this prohibition only applies in a place where there are no people around at all, for instance, if one is walking through a forest and the like; however, within the city where people are present, it is permissible to walk around alone. (There are those who rule stringently even within the city, but the custom is not so.)
Similarly, one must take care not to perform any dangerous activities during the time “Ketev Meriri” is present. The Midrash and Poskim speak lengthily about this issue.
The Poskim write that one should not reveal the reasons of the Torah, such as these, in public; we are even warned not to reveal the reasons for the decrees of our Sages in public, for there may be ignoramuses who will mock ideas that seem foreign to them. We have nevertheless discussed the idea of the presence of “Ketev Meriri,” for this is a matter which may lead to danger and it is proper to warn the public about it.