When do Harry Potter and Ginny Weasley kiss for the first time?

Harry and Ginny kiss for the first time in Harry Potter and the Half-Blood Prince, during the Battle of the Astronomy Tower.

It happens after Harry has just seen Dumbledore die and is feeling incredibly emotional and vulnerable. Ginny, who has been watching the fight unfold, finds him and comforts him. Their shared grief and connection lead to the kiss.