1) One may not let blood on the eve of Shavuos, nor may one donate blood or have an operation which is not urgent on this day, as it is a dangerous day, as if KLal Yisrael would not have accepted the Torah, they would have all been killed. Because of this, Chazal decreed that one may not let blood even on the eve of the other festivals (in a time of great urgency one should ask a wise man). (Shulchan Aruch and Mishne Berurah 468, Tzitz Eliezer 12,46)
2) On the night of Shavuos there is a custom to start the evening service late, as it says about the counting of the Omer (seven complete weeks”, and there is therefore a necessity that the days of the counting of the Omer should be complete, and one does not start the evening service until it is definitely night. (Shulchan Aruch and Mishne Berurah 494)
