http://revach.net/article.php?id=2426

Section: Moadim   Category: Around The Year
Chodesh Tammuz: Bnei Yisaschar The Toughest Month Of The Year
The Name of Hashem (Yud, Hey…) is a name of Rachamim, mercy.  There are 12 combinations of these letters that represent the Shem Hashem in different degrees of Rachamim and Din.  Each varying combination corresponds to a particular month of the year.  For the Month of Tammuz the letters of the name of Hashem (Tzirufei Osios) are written in reverse order (Hey, Vav, Hey, and Yud).  This backwards spelling represents Din.  In this case the Shem Hashem, which represent Rachamim, is very weak because of the harsh Din hovering over this month.

This remez for this spelling is found in the words of the Megila where Haman bemoans Mordechai’s lack of respect and says, “VChol Zeh Einenu Shoiveh Li”, all his greatness and wealth is worthless because Mordechai refuses to bow to him.  The last letters of “Zeh Einenu Shoiveh Li” are Hey, Vav, Hey, and Yud.  This is the source for this Tziruf of the name of Hashem.  The Bnei Yisaschar says that this utterance of Haman, had hidden in it Hashem’s Middas HaDin, which hung over him at that point and caused his downfall.

Although the three weeks starts only from after the middle of the month (17 of Tammuz), the Bnei Yisaschar explains that the whole month is Middas HaDin since the Meraglim were in Eretz Yisroel every day during this month.  On the other hand, Av, only has the Middas HaDin during the first half of the month.