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"Once a Day" is a song written by Bill Anderson and recorded as the debut single by American country artist Connie Smith. It was produced by Bob Ferguson for her self-titled debut album. The song was released in August 1964, topping the Billboard country music chart for eight weeks between late 1964 and early 1965. It was the first debut single by a female artist to reach number one on the Billboard Hot Country Songs. This song peaked at number one for the week of November 28, 1964, and it stayed at number one for eight consecutive weeks, a record for a female solo artist for nearly 50 years, until it was surpassed by Taylor Swift's "We Are Never Ever Getting Back Together" in December 2012.

In 2021, "Once a Day" was deemed "culturally, historically, or aesthetically significant" by the Library of Congress and selected for preservation in the National Recording Registry.