festive dress; ceremonial kimono
Often refers to a baby's first formal kimono worn for shrine visits or celebrations, but can also mean festive attire in general.
初宮参りに赤ちゃんに祝い着を着せた。
We dressed the baby in a ceremonial kimono for the first shrine visit.
お正月には祝い着を着て親戚に挨拶に行く。
On New Year's, we wear festive clothes and go greet our relatives.
Compound of 祝い (celebration) + 着 (clothing). The exact historical origin is uncertain, but the word has been used for festive garments, especially for infants, since at least the Edo period.