From Glossary of Meteorology
(Also called day of rain.) In British climatology, a period of 24 hours, normally commencing at 0900 UTC, in which at least 0.01 in. or 0.2 mm of precipitation is recorded.
It is the practice of the U.S. National Weather Service to use phrases that are more self-defining, such as "day with measurable precipitation" or "day with 0.01 inch or more" (of precipitation).