I watched this series on Swedish television in the early 1980's maybe late 1970's. Maybe the series was French but very unsure. I don't remember much, only that a person was killing off a number of people, and I think it was on the same date every year. I only remember one scene clearly, it was on a large cruise ship and a woman terrified of being this year's victim returns to her cabin at midnight relieved she has not been killed during the day, but she has forgotten the ship has crossed a time zone (the dateline?) and is killed in her cabin.