Flight Sub-Lieutenant Harold Day DSC (17 April 1897 – 5 February 1918) was a Welsh World War I flying ace credited with 11 confirmed aerial victories.

8 Naval Squadron and flew the Sopwith Camel for his remaining ten victories.

During December 1917, he teamed with Guy William Price to drive down DFW two-seater reconnaissance aircraft on three different days.

On 2 February 1918, he joined Robert J. O. Compston and three other RNAS pilots in destroying a German reconnaissance aircraft, and in driving down an Albatros D.V later in the day.

He is buried at St. Mary's ADS (Advanced Dressing Station) Cemetery, Haisnes, France.