ダービー is the transliteration of derby, referring to the horse race 東京優駿 (Tokyo Yushun), aka Japanese Derby, that took place on May 29.

As mentioned in a post before,

…Horse racing is quite popular in Japanese society (partly due to the gamble/lottery related), and inspired subculture projects such as the phenomenal ウマ娘 プリティーダービー (Uma Musume Pretty Derby)…

Netizens in Japan are having heated discussion about the race on Twitter, causing the tag to trend.

Learn more on wikipeida about Japanese Derby.