520 (wǔ èr líng) sounds like 我爱你 (wǒ ài nǐ, 'I love you'), so May 20th became an internet-era Valentine's Day in China. Sending a 520元 red envelope on WeChat, or a 1314 ('一生一世' = forever), are the classic digital love gestures.
mistakes
Read the digits individually (wǔ èr líng), not as 五百二十. The numeric amount is read normally: 520元 = wǔbǎi èrshí yuán.