I like how those were happy days feel wistful.

 Those were happy days. It is a charming song with the sentiment of looking back on past love and recalling happy memories.
The band sound, which is bright yet somewhat wistful, makes the song's atmosphere even more special.
The lyrics feel so warm to me because they convey the feeling that the time spent together was precious even after the breakup.
The melody flows naturally, so even though I listen comfortably, at some point I find myself focusing on the lyrics.
If you are looking for an emotional DAY6 song that brings back memories, I would like to recommend this.