曰only appear in classical Chinese and ancient poetry, such as《论语》《孟子》《倬彼我系》（曰天曰人，是祖是考。） 说is commonly used on our daily basis

曰 is the archaic form. It is used in classical Chinese, for example 子曰 :「吾 未 见 刚 者 。」或 对 曰 :「申 。」子曰 :「申 也 欲 ,焉 得 刚 」 It is not really used in modern Chinese except when referencing classical Chinese.