There are four days in a Jewish man's life that are his important days of all, and each of these days is represented in one of the four letters of Hashem's Most Holy Name.
Hashem's most holy name is Yud Kay Vav Kay.
The first most important day in a Jewish man's life is the day of his bris, his circumcision. This is his entry into Hashem's covenant with the Jewish people. This act is associated with the first letter of the Name, the Yud, which resembles the place of the bris.
The second most important day in his life is the day of his bar mitzvah. This day marks his spiritual maturity when he assumes responsibility for the commandments. This day is represented by the first letter Kay in Hashem's Name which stands for Binah (understanding).
The next most important day in his life is the day of his marriage. This is represented by the letter Vav which is the Yud pulled down into the lower world. The Vav looks like a Yud that has been extended downward, which for the groom represents marital relations and the fulfillment of the mitzvah of having children.
The fourth most important day of his life is the day of his passing away. This day is represented by the fourth letter in the most holy Name, the final Kay which stands for Malchus, Kingship. This is the day when his soul goes up and stands for judgment before the Holy King.
These days are his most important and most holy days. They should be spent with great awe, and tremendous joy as Hashem's Presence is only experienced in joy.