蜀犬吠日

shǔ quǎn fèi rì

lit. Sichuan dogs bark at the sun (idiom); fig. a simpleton will marvel at even the most common things

alludes to the Sichuan foggy weather where it's uncommon to see a sunny day

shǔ

short name for Sichuan 四川[Si4 chuan1] province

quǎn

(literary) dog

fèi

to bark

abbr. for 日本[Ri4 ben3], Japan