The city of Crestview plans to commemorate the 75th anniversary of   D-Day. The free observance is 4:30-8 p.m. June 6th at Warriors Hall, Whitehurst Municipal Building, 201 Stillwell Boulevard, in Crestview.    The event will also feature an essay contest, with north Okaloosa County students in grades 7-12 invited to submit essays of 500 words or less on the topic, “The Importance of Commemorating D-Day: June 6, 1944.”   First prize is $500 for high school and $300 for middle school winners.