Midsummer Dayn.1. June 24, observed in Europe, Latin America, and Scandinavian communities in the United States in commemoration of the summer solstice.2. June 24, observed in many Christian churches in commemoration of the birth of Saint John the Baptist.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.Mid?summer Day? n. Chiefly Brit. the saint’s day of St. John the Baptist, celebrated on June 24. [before 1150] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.