Auspicious Day
Lucky Day
A day you hold in high esteem is an auspicious day. That's a fancy way of saying it's a really lucky day!
かじつ
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
本日は、皇太子殿下御生誕の嘉日なり。
It’s the auspicious day of the Crown Prince’s birth today.