CELEBRATION' (4 letters) A landmark occasion is an important event that marks a significant MILESTONE or CELEBRATION. The term 'landmark occasion' can refer to a variety of events, such as a birthday, wedding, anniversary, or other special occasion. The two crossword solutions for this term are 'MILESTONE' (9 letters) and 'CELEBRATION' (4 letters). A MILESTONE is a significant event or accomplishment that marks a major turning point in one's life, while a CELEBRATION is a joyous gathering of people to commemorate an event or achievement.