What is the Ketav Meriri?
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Let's back up a minute and start with a simple understanding.
In Ha'azinu (Deuteronomy 32:24), God warns of:
What's this "ketev mriri"?
To quote R' Aryeh Kaplan's Living Torah:
It's generally known that the Three Weeks are considered an inauspicious time; ask your local rabbi exactly what risky activities you should therefore curtail during this period.
Beyond that, if a good Jewish person assumes that halacha is what made it into the Rambam and Shulchan Aruch, and none of the demon stuff appears there, so what to make of the aggadic spooky stuff in the Gemara? I'm sure there's a deeper meaning, but for now, Teiku.
Or best of all, keep the Torah and mitzvos, so we don't get any of the punishments warned about.
Rav Yitzchak Meir Morgenstern in Nishmatin Chaditin on Vayishlach 5773 section 40 writes (my own translation)
It is a dangerous spirit that is active in between 17 of Tammuz and Tishabav (according to one opinion) from the 4th hour until the ninth hour of the day(summer time around 11-2). Which is one of the reasons to not do things that are potentially dangerous during the three weeks. This includes things like: hitting children(beis yosef 551) Walking by yourself(Beis Yosef 551),Rav Chaim Kanievsky says even to do a mitzvah (Nechmas Yisroel pg.58, footnote 161) and you should not walk between the sun and the Shade(Mishnah Berurah 551.102) He is described as being full of eyes scales and ears and rolls like a ball between the sunlight and the shade, Reb Shimon Bar Yochai says he has one eye on his heart and who ever sees him dies on the spot.
For more extensive research see: Pesachim 111b The disscusin includes the Morning Ketev is the"Ketev Meriri". The Afternoon Ketev is the "Ketev Yashud Tzaharayim,". and times of the year when Ketev are found and where he lives.
Intresting Fact from Rashbam,Pesachim 111b "Shaata": Rich people are at less risk of harm from Ketev, because their Mazal is good