Blood-letting was a common medical practice at the time. Physicians believed that removing blood could be beneficial to health. The objective was to balance the humours (blood, phlegm, black bile and yellow bile) as an excess of one humour over another was thought to cause poor health. Blood was associated with Spring, and so excesses were expected in this season.
There are parallels in other medical traditions, including Ayurvedic and Chinese medicine, which also stress internal balances.