The 24th wedding anniversary is traditionally called the musical instrument anniversary. This is not considered one of the more major anniversaries, and it is the 25th anniversary — the silver anniversary — that is considered a more significant milestone. The name of the anniversary traditionally indicates what gift should be given.
The gemstone for the 24th anniversary is tanzanite, and lavender is the traditional color. A music-themed party would be appropriate, and gift ideas may include things like vintage sheet music, instruments, popular music from the couple's dating years, prints or posters of famous musicians and even technology such as speakers or personal music devices.