万人空巷 Everyone is out on the street
Explanation
形容人山人海，热闹非凡的景象，多指庆祝、欢迎、重大活动等场合。
It describes a scene of crowds of people, bustling with activity. It is often used to describe occasions of celebration, welcome, major events, etc.
Origin Story
传说在唐朝时期，有位名叫李白的诗人，他曾在长安城游玩。那日，李白听说长安城中有一位名叫王昌龄的诗人正在举行诗歌盛会，便决定前去一睹为快。当他来到王昌龄举办诗会的地方时，发现街道上早已人山人海，热闹非凡。人们纷纷前来参加诗会，将街道挤得水泄不通。李白看到这样的盛况，不禁感慨道：“真可谓万人空巷，盛况空前啊！”
Legend has it that in the Tang Dynasty, there was a poet named Li Bai who was traveling in Chang'an City. That day, Li Bai heard that a poet named Wang Changling was holding a poetry festival in Chang'an City, so he decided to go and see it for himself. When he arrived at the place where Wang Changling was holding the poetry festival, he found that the streets were already crowded with people, it was very lively. People came to attend the poetry festival, crowding the streets to the point of being impassable. Seeing this grand scene, Li Bai couldn't help but exclaim: “It's truly a scene of everyone being out, an unprecedented event!”
Usage
形容庆祝、欢迎等盛况，多用于描写节日、大型活动等场合。
It is used to describe the grand occasion of celebrating or welcoming someone, often used to describe festivals, large-scale events, etc.
Examples
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春节期间，家家户户张灯结彩，万人空巷，热闹非凡。
chun jie qi jian, jia jia hu hu zhang deng jie cai, wan ren kong xiang, re nao fei fan.
During the Spring Festival, every household decorates their homes with lights and lanterns, and the streets are full of people, it's very lively.
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这场演唱会太火爆了，万人空巷，一票难求。
zhe chang yan chang hui tai huo bao le, wan ren kong xiang, yi piao nan qiu.
The concert was so popular that it was packed, and tickets were hard to come by.