Why is today Valentine's Day?

The seventh day of the seventh lunar month is the Beggar's Day, which is also called Valentine's Day in China.

Supplement:

The 24 solar terms in China are basically related to agricultural production. Many far-fetched legends of the Cowherd and the Weaver Girl are just the subject matter of old people telling stories and literati expressing their feelings. In ancient China, men plowed and women wove, and the seventh day of the seventh lunar month was the harvest season, which meant the grain settlement in the second half of the year. After busy farm work in the fields, farmers began to celebrate the harvest and set off lanterns and firecrackers to treat themselves.