If you can congratulate someone for an occurrence in their life, it must be an auspicious event. You'll remember this happy event for a long, long time.

慶事 is a very formal word used to refer to an auspicious event, such as a wedding, the birth of a child, or other significant celebrations. You might encounter 慶事 in official announcements or formal congratulatory messages.

This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.