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9. Blood letting-Blood tests and other blood work:[1]

It is customary of all Israel, due to a decree of the Sages, not to let blood on any Erev Yom Tov [from daybreak], and one may not swerve from this custom, and it is hence forbidden to do so.[2] However, [with exception to Hoshana Raba] this only applies to real forms of bloodletting that release much blood, however, those forms of bloodletting that do not release much blood, known as Kep Zetzin, may be done on Erev Pesach and Erev Sukkos.[3] [Thus, one may have a blood test, or other form of blood work, done on Erev Yom Tov, with exception to Erev Shavuos and Hoshana Raba, unless it involves a case of danger.[4]]

The night before Yom Tov:[5] It is permitted to let blood on the night before Erev Yom Tov, with exception to Hoshana Raba as will be explained.

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[1] Admur 468:22; Rama 468:10; Mahariy Viyal 19

[2] The reason: As on the day of Shavuos, before Matan Torah, a spirit called Tavuach came and said that if the Jewish people do not accept the Torah he will slaughter them, their blood and flesh. Therefore, there is an everlasting danger in every generation to let blood on this day, which is Erev Shavuos, and the sages decreed against doing so on any Erev Yom Tov, due to Erev Shavuos. [Admur 468:22; Shabbos 129b; M”A 468:15; Taz 468:7; Darkei Moshe 468:3; Kol Bo 58 in name of Maharam] 

[3] Admur 468:23; M”A ibid in name of Maharil

The reason: As these days are only forbidden in bloodletting due to a mere decree. [Admur ibid]

[4] Nitei Gavriel 8:3 based on Admur 468:23

[5] Admur 468:23; M”A 468:15