A leading writer of the Tang Dynasty (táng cháo 唐朝,618-907AD) named Liu Zongyuan (liú zōng yuán 柳宗元),once told a story concerning an interesting phenomenon in Sichuan Province. In this basin area, you can hardly saw the sunshine since it was cloudy and foggy most of the year on account of its landform. Therefore, as long as the sun came out, the local dogs would become rather curious and exciting,barking loudly to the sun.

This idiom is used to indicate being surprised at something normal, due to one's ignorance.

唐朝时期著名的文学家柳宗元，曾经讲过一个“蜀犬吠日”的故事。在中国西南地区的四川省，那里的气候多雨多雾，有时阴雨连绵、雾气蒙蒙长达数月，一年四季很难看见太阳，结果那里的狗一见到太阳出来，都感到很惊奇，就大声狂叫起来。

“蜀犬吠日”这个成语比喻少见多怪，对本来很平常的事情，由于见识短浅而感到奇怪。

蜀犬吠日 (shǔ quǎn fèi rì)
【翻译】A Sichuan Dog Barks At the Sun.
【释义】蜀：四川省的简称；吠：狗叫。原意是四川多雨，那里的狗不常见太阳，出太阳就要叫。比喻少见多怪。
【例句】这种事情经常发生的,可他却表现得非常惊讶，真有点蜀犬吠日。
【近义词】少见多怪、孤陋寡闻
【反义词】见怪不怪、司空见惯