day of the Horse
Refers to a day in the traditional Japanese calendar associated with the Horse zodiac sign. Often used in the context of festivals or rituals, such as the first day of the Horse in February (初午).
Hatsu-uma is the first day of the Horse in February.
Compound of 午 (uma, 'Horse', the seventh sign of the Chinese zodiac) and 日 (hi, 'day'), with the genitive particle の. The reading うま is the native Japanese name for the Horse zodiac sign.