The Hebrew alphabet is very unique.
One element is that letters also stand in for numbers.
Aleph (א) = one
Bet (ב) = two
. . . .
Chet (ח) = eight
Tet (ט) = nine.
Now, there are some words that also sound like a letter. In our case, the letter chet (ח), sounds like the Hebrew word for 'sin', 'transgression' which is properly spelled out as חטא.
So here's the poster (explanation below)
The theme of the Ellul Bet Medrash activity is "The Chet Before the Tet" which can mean the 8th before the 9th, or, and this is the subliminal intention, the sin before the Ninth Day (of Av)!
Very nicely done.