The Gemara in Masechet Shabbat (p.129) states that on Ereb Shabuot, one should refrain from bloodletting, because of a Shed called "Taboach" that poses a threat, in an attempt to prevent us from receiving the Torah on Shabuot. The Kol Bo cites the MaHaram MiRotneberg (Rabbi Meir of Rothenberg, 1220-1293 Germany) that this restriction was also extended to include all Ereb Hag, not just Shabuot. The Rama in Hilchot Pesah (468:10) adopts this position. However, Rambam and Maran do not bring this Halacha. The Peri Hadash (Rav Hizkiya Da Silva, 1656-1695), as well as the Mateh Moshe, say that today we are not concerned with the threat, and "Shomer Ptaim Hashem" (God will guard the foolish).
The contemporary application of this issue is with regard to elective surgery, donating blood and even blood tests. If there is a medical need to perform the procedure, everyone agrees that it is permitted.
The book Rofeh Holim (3:148), raises the question whether the restriction applies in a situation like this year, in which Ereb Shabuot is Shabbat. Is it permitted to do such procedures on Friday? He rules that the restriction does apply, and one should refrain from elective surgery, donating blood and blood tests on Friday. He cites the MaHari Bruneh (Ch. 118), Rav Haim Palachi, Kaf HaHayim (468:98) and the Munkatcher Rav who all rule this way.
SUMMARY
One should refrain from elective surgery, Donating blood and blood tests on Ereb Shabuot, which this year includes the Friday before Shabuot.
