na-adjective
tranquil; calm; peaceful; serene
Describes a scene, atmosphere, or feeling that is gently calm and undisturbed. Often used for weather, landscapes, or a relaxed mood.
See also: のどか
今日は風もなく、のどやかな一日だった。
Today was a calm day with no wind.
のどやかな田園風景が広がっている。
A tranquil rural landscape stretches out.
のどか is the more common and standard word with the same meaning. のどやか is a less frequent variant, slightly more literary or old-fashioned.
The exact derivation is uncertain. It is likely related to the more common のどか, with the suffix やか added to form an adjectival noun.