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The G-dly revelation of Shavuos:
On the two days of Shavuos the same revelation that was revealed by Matan Torah is revealed.
The Satan has no power on Shavuos-A time for good resolutions:
In a letter written to Yeshiva students [of all affiliations] in the Unites States and Canada, the Rebbe Rayatz wrote as follows: On the two days of Shavuos [in the Diaspora, the first day in Eretz Yisrael], the Satan is distracted to the same extent as he is distracted on Yom Kippur, and by the Tekios of Rosh Hashanah. Hence, the Satan does not have the ability to persecute the Jewish people on this day. For this reason, Shavuos is a very auspicious time for one to learn Torah and perform Teshuvah and service of G-d without distraction from the evil inclination. It is similar to the auspiciousness of Yom Kippur and Rosh Hashanah during Tekios. The Rebbe requested that this matter is to be publicized to the masses.
Receiving the Level of Taanug in Torah and Mitzvos:
On Shavuos, the level of Divine pleasure in each Mitzvah is revealed. The common denominator of all Mitzvos is that they are a revelation of G-d’s will. Each Mitzvah that one performs connects him with G-d and hence, with this knowledge, one should rejoice to no bounds upon performing the Mitzvah. In the word of the Sages “A light Mitzvah is to be in your eyes like a severe Mitzvah”. Nevertheless, each Mitzvah contains a unique Divine intent and purpose, and hence there is a difference in the effect of Mitzvos on the Seder Hishtalshlus, the Divine chain of worlds. In truth, the reason for this differentiation is due to the root of the Mitzvos in G-d’s Taanug, essential pleasure. The Taanug of Hashem is different by each Mitzvah, with each Mitzvah holding a different Divine pleasure for G-d. This Taanug is an infinitely greater level than is the Divine will for the Mitzvos, and it is in fact this personal pleasure involved in each Mitzvah which created the level of will, for G-d to will their performance and command them. On Shavuos this level of Divine pleasure in each Mitzvah is revealed.
 Igros Kodesh Rebbe Rayatz 5 p. 76
 Igros Kodesh Rebbe Rayatz 5 p. 76; Hayom Yom 3rd Sivan and 4th Sivan based on Tosafos Shabbos 89a; Otzer Minhagei Chabad p. 290
 See Tosafos Shabbos 89a
 Tosafos Shabbos 89a
 Otzer Minhagei Chabad ibid
 Likkutei Torah Bamidbar p. 18