To differentiate this from 日, note that this character is much wider.
Mnemonic
[yu-] Yugi is in [1] front of the [-e] electricity plant where suddenly the 日 sun comes down to him and expands horizontally, becoming fat, and slowly 曰 says a few strange words: "ONLY WHEN I'M HORIZONTALLY WIDER CAN I 曰 SAY THINGS". Yugi covers his eyes with one of his cards.