Tosafos “keiylu” (second on amud beis) discusses certain features of the Jewish calendar, including that Pesach never falls out on Friday, which means Shavuos never falls out on Shabbos.

Peri Latzaddik (Sefer Kedushas Shabbos 3) notes the dichotomy that the first Shavuous, when the Torah was given actually occurred on Shabbos, never again to be repeated. What is the significance of this?

He explains that the way of the world is one must work in order to reach a spiritual attainment. Therefore, on a material level one must work all week physically in order to enjoy Shabbos. On a spiritual level to comprehend the “raw” written Torah, one must toil all week in the study of Torah, that is the Oral Torah to benefit from the spiritual gifts of understanding the Torah. The six days of the week represent the six orders of the Mishna. On the other hand, the first Shabbos of the giving of the Torah corresponds with the first Shabbos of creation. This first Shabbos is a free gift to get humanity started but from then on the work begins to earn the Torah or the world.

Nothing in this world, physical or spiritual can be attained without the work. We are given the initial gift of life, intellect and heritage that we are born into, but the rest we make for ourselves. 

Another interesting historical note that ties into toiling in Torah during the week but resting on Shabbos is found in a fascinating responsa of the Ramah (#7). The Ramah defended his study of philosophy and Aristotle using several arguments: 1. He only studied parts of Aristotle that were vetted and approved by Rambam as correct in his Moreh Nevukhim. 2. He attained knowledge of Shas and poskim prior to his philosophy studies. 3. Studying the physical and biological sciences enhances appreciation for G-d. 4. He only studied philosophy on Shabbos, Yom Tov and Chol HaMoed, leaving intense study of Gemara and halakha for weekdays.

By the way, the Ramah lived only until age 42.  How does one accomplish that much in such a short lifetime???