The single was released in two editions: a regular edition containing six tracks which includes the instrumental versions, and a limited edition containing the title track and a DVD of the music video. "Beautiful Days" was the group's fourth single to be released in 2008, the most number of singles released in a year for the group. The release date and announcement of the single was first confirmed on the online version of the Japanese Yomiuri news on September 21, 2008 and was first heard on the TBS television program Mino Monta no Asa Zuba (みのもんたの朝ズバッ！), a day later, as promotion for Ryūsei no Kizuna. The single was available for pre-order on online shopping sites on September 27, and the full song was heard for the first time on October 5 on Ninomiya's radio program, Baystorm. A short version of the music video was also aired on M-On before live promotions of the song, which started on the music program Hey! Hey! Hey! Music Champ on the November 3rd, which was the only television promotion before the single's release.
The single was listed on Japan Gold Disc Award's list of The Best 10 Singles of 2009. It was also named Best Theme Song in the 59th Television Drama Academy Awards.
The single debuted at the top spot on the Oricon daily singles chart with total index sales figure of 83,052 (an estimated 145,341 copies sold on the first day), making it the group's 13th straight number one release and their 21st number one overall with a total of 351,860 copies sold in the first week. "Beautiful Days" also topped the ranking for music program Music Station's Ringtone Ranking.
By selling a total of 421,876 copies by the end of 2008, the single emerged as the tenth best-selling single of the year in Japan. The single is certified Double Platinum by the RIAJ for shipment of 500,000 copies.