H Day
H Day or H. Day may refer to:

 Dagen H (H Day), the day when traffic in Sweden switched to driving on the right
 H-dagurinn (H Day), the day when traffic in Iceland switched to driving on the right
 H. Day, a penal administrator of a convict settlement on Norfolk Island
 H Day, July 19, a holiday commemorating the day Pittsburgh won a 1911 court case to return the final H to the Name of Pittsburgh.