What is the meaning of the idiom "a gala day"?

A day of happiness.

The correct answer is A day of happiness. The idiom "a gala day" means a day of celebration or festivity. It is often used to describe a day when there is a special event or occasion, such as a national holiday, a sporting event, or a cultural festival.

The other options are incorrect. A day of sorrow is the opposite of a gala day. A routine day and an uneventful day are both days that are not particularly special or noteworthy.