Steeping the spearmint in bourbon gives this timeless drink a big flavor boost. Start this mint infusion up to a week in advance.
Combine bourbon and mint in a large pitcher, cover, and refrigerate 12 hours or up to 1 day. Strain, discarding mint. Add sugar, stirring to dissolve. Divide julep mixture among 4 ice-filled glasses. Garnish each with a mint sprig.
The strained mint-infused bourbon can be refrigerated for up to 1 week in advance.