In ancient times, the four great happy events refer to meeting the rain after a long drought, meeting an old friend in another country, spending the night in the bridal chamber and becoming the number one scholar.
Drought meets rain: after a long drought, suddenly there is a good rain. Describe the long-awaited joy of finally getting what you want.
Meet an old friend in a foreign land: I met an old friend who is far away from home. Something that makes people happy.
Wedding night: Wedding night.
Time to be the number one scholar: refers to the time to win the number one scholar in the imperial examination.