Today is the fifth day of the fifth month in the Chinese calendar. This marks the day of 端午 (Duānwǔ), one of the most important dates of the Chinese year.
端午 is also known in English as the Dragon Boat Festival, but as pulchritude explains in the post linked below, this name captures only one aspect of what is actually a multifaceted occasion. So it's worth knowing how to pronounce the Chinese name; see Forvo for an example pronunciation.
The main theme of 端午 is health/hygiene/purification, and celebration of the day includes several related customs such as drinking realgar wine, the symbolic destruction of the "five poisons", and the use of various herbs. Dragon boat racing is another custom associated with 端午; teams of paddlers race in long boats, kept in synchronisation by a drummer who sits at the head of the boat. Finally, 粽子 (zòngzi) are perhaps the most famous of the foods associated with the day; these are glutinous rice dumplings filled with various morsels and wrapped in bamboo leaves.
Aside from eating 粽子, the dragon boat races are the only part of 端午 that I've actually experienced (and then only as an onlooker)[see footnote], so here are some links which hopefully give a fuller picture:
- 端午, or the Fifth Day of the Fifth Month by pulchritude
- Duanwu Festival on Wikipedia
- Folk Customs of the Dragon Boat Festival from the Sinolingua Kaleidoscope of Chinese Culture
- Grass Fighting on Dragon Boat Festival (I suspect this may be machine-translated from a Chinese source)
Footnote:  Fellow Londoners may wish to note that this year's dragon boat races will be held on Sunday 19 June at the London Regatta Centre (near Royal Albert DLR station on the Beckton branch). I might be going — haven't decided yet.