May 4th; specifically refers to the May Fourth Movement of 1919 which began as a protest against the Treaty of Versailles

Youth Day; in mainland China, it is celebrated on May 4th to commemorate the 五四运动 wǔ​sì​yùn​dòng of 1919; in Taiwan, it is celebrated on March 29th to commemorate the 1911 Huanghuagang Uprising.