Why do Shuls put up notices saying משנכנס אדר מרבים בשמחה ?
Do we think that a notice will change our mood? And if so, why not put up a notice in Elul saying that the Yom HaDin is coming?
The Nitei Gavriel - Purim Perek 11 #4 brings from the Yalkut Avraham Siman 686, that the Minhag was to put the משנכנס sign over the Zecher L'Charban area, since it is a Zeman Simcha.
Regarding hanging a Mishenechnas sign in the Shul - see page 262 - that the Minhag was to hang it on the Western wall of the Shul - where often that is where the entrance is.
Why we do not hang up in Elul signs - the שו"ת אפרקסתא דעניא ח"ב או"ח סי' לו ס"ק ד says that since there is no such Maamar Chazal therefore we do not do it.