Shavuos...

...is an auspicious time Above.
On that day Hashem confuses the "supernal accuser" of Israel, a.k.a the satan, similar to His confounding the accuser during Shofar - sounding on Rosh Hashana and on the holy day of Yom Kippur.
So Shavuos is an opportune time to achieve everything in improving Torah-study and Avoda marked by fear (awe) of G-d, and also to strive in Teshuva concerning Torah-study, without interference by the accusing Satan.