'세배'is special type of '절'.
Having '절' at '구정'(Jan first in luna calendar) is called '세배'.
So I'll interpret 'deep bow' as '절' from now on.
Here is description for '세배' in korean dictionary.
아침 일찍이 남녀노소가 모두 새옷(설빔)으로 갈아입고, 차례를 지낸 뒤에 자리를 정리하여 앉는다. 그리고는 조부모·부모에게 먼저 절하고, 형·누나 등 차례로 아랫사람이 윗사람에게 절을 하여 새해 첫인사를 드린다.
그리고 차례를 지낸 설음식으로 아침식사를 마친 뒤에는 일가친척과 이웃어른을 찾아가서 세배를 드린다. 요즈음은 또 직장 관계로 해서 회사 사원들은 회사 윗사람을 찾아가서 세배를 드리기도 한다.
세배를 드려야 할 어른이 먼 곳에 살고 있을 경우, 정월 15일까지 찾아가서 세배하면 예절에 어긋나지 않는 것으로 되어 있다. (정월 15일 means january 15th in luna calendar)
But nowadays, not many people have deep bow to their boss or every elders nearby. Usually do only to their relatives.
In wedding, bride and groom have deep bow to their parents, elder relatives. Some couples allot this as a part of the wedding, some couples don't. But having deep bow before/after wedding ceremony is usual(especially man bow to his opposite elder relatives).
And '제사' is another example. '제사' is ceremony for ancestors of family. burn a incense and display some foods and have deep bow three times, or five times.... i'd heard there are certain rules. But nowadays, many family don't have '제사' anymore.
In korea, there is another new years day which is not January first in ordinary calendar. It's January first in luna calendar, called '구정'. (In 2017, it's January 28th)
(Jan first in ordinary caledar is called '신정')
In '구정', child and teenagers earn money for having deep bow to their parents and relatives(this money is called '세뱃돈').
Of course, adults bow to their elders too. But usually don't get '세뱃돈'.
Some kids earn ₩750,000 for having bow to his or her every relatives. Usually kids earn ₩30,000 to ₩200,000. (And most of the money goes back to their parents)
Anytime if you want to express your deep respect and happiness, or want to make a 'great good-bye' or 'massive glad-to-meet-you' or 'big thank-you' in traditional ways, you can have a deep bow to your elder relatives, or someone you owed a lot.
For example, If you are going to the war, you can have a deep bow to your parents instead of saying 'good bye'.
Same for getting back from the war.
(₩ is Korean currency, pronounce 원 'won' / $1 == ₩1,000)