Three weeks are period of danger from demons

 Eichah Rabbah (01:29). Within the days of distress from the seventeenth of Tammuz to the ninth of Ab, during which Keteb meriri is prevalent The demon Keteb stalks through the greater part of the midday period, from the beginning of the sixth hour until the end of the ninth. R. Levi said: It spoils the course of the day from the end of the fourth hour until the beginning of the ninth; and it does not walk in the sun or shade but in the shadow near the sun. R. Johanan said: It is all over full of eyes, scales, and hair. R. Simeon b. Lakish said: It has one eye set over its heart and whoever looks at it falls down dead. Once a pious man looked at it and fell dead upon his face, and some say that it was R. Judah b. Rabbi. Samuel saw it but did not fall; for he said, ‘It is the snake of the house.’

Pesachim (111b) Seeing a Keteb Meriri approaching him on the left, he transferred R. Papa to his left and R. Huna son of R. Joshua to his right. Said R. Papa to him: ‘Wherein am I different that you were not afraid on my behalf?’ ‘The time is in your favour,’ replied he.   From, the first of Tammuz until the sixteenth they are certainly to be found; henceforth it is doubtful whether they are about or not, and they are found in the shadow of hazabe which have not grown a cubit, and in the morning and evening shadows when these are less than a cubit [in length], but mainly in the shadow of a privy.

Pesachim (111b) Keteb Meriri: there are two Ketebs, one before noon and one after noon; the one before noon is called Ketheb Meriri, and looks like a ladle turning in the jug of kamka. That of the afternoon is called Keteb Yashud Zaharaim [Destruction that wasteth at noonday]; it looks like a goat's horn, and wings compass it about. 

  Vayikra Rabbah (12:3) Our Rabbis explain that ‘keteb’ is a demon. And why was he called keteb’?’ R. Abba son of Kahana said: Because he breaks into the daily studies from the beginning of four hours of the day to the end of nine. R. Levi said: Because he robs pupils of their noonday lessons from the end of four hours to the beginning of nine hours. He holds sway neither in the shade nor in the sun but between the shade and the sun. His head is like that of a calf and a horn grows out from the centre of his forehead, and he rolls like a pitcher. R. Huna, in the name of R. Joseph, said: ‘Keteb meriri’ is in form covered with scales, hairy all over, and full of eyes. And, said R. Simeon b. Lakish, he has one eye set in his heart, and anyone who sees him can never survive, whether it be man or beast. Anyone who sees him drops down dead. ‘Meriri’ holds sway during the period between the seventeenth of Tammuz and the ninth of Ab. Hezekiah saw him and dropped down on his face and died. R. Phinehas the priest said: It is related that one saw him and was stricken with epilepsy They reported that Judah b. R. Samuel saw him and did not drop down. Nevertheless, it is reported that he died.

Bamidbar Rabbah (12:03) Our Rabbis explain that ‘keteb’ is a demon. And why was he called keteb’?’ R. Abba son of Kahana said: Because he breaks into the daily studies from the beginning of four hours of the day to the end of nine. Levi said: Because he robs pupils of their noonday lessons from the end of four hours to the beginning of nine hours. He holds sway neither in the shade nor in the sun but between the shade and the sun. His head is like that of a calf and a horn grows out from the center of his forehead, and he rolls like a pitcher. R. Huna, in the name of R. Joseph, said: ‘Keteb meriri’ is in form covered with scales, hairy all over, and full of eyes. And, said R. Simeon b. Lakish, he has one eye set in his heart, and anyone who sees him can never survive, whether it be man or beast. Anyone who sees him drops down dead.