今天和今日意思是一样的means today 这天和这日意思是一样的means someday

今天，在口语和书面语中经常出现。 今日，在生活中很少出现。 今天，今日都可翻译成today。 这天，任何一天都可以称为这天。 这日，在评书中经常出现。 这天，这日都可翻译成this day,a day。 今日，这日经常出现在文言文中，也就是中国古代文学。 今天 it often occurs in both spoken and written language. 今日 rarely in life. 今天，今日can be translated into today. 这天 any day may be called this day. 这日 often appear in storytelling. 这天，这日can be translated into this, day, a, day. 今日，这日often appears in classical Chinese, that is, ancient Chinese literature.

“今天” 和“今日” 都是today的意思，不过 “今日” 比较formal，一般用作书面语。 “这天” 和“这日” 都是that day的意思，不过 “这日” 比较formal，一般用作书面语。

今天 is the correct and the most commonly used one. 今日 is a very formal and an archaic word for "today". 这天，这日 do not exist.

今天 = 今日 = today. Using 日 is more formal, poetic and a preference in use in southern China. Like in all languages, using the formal word conveys your tone, emphasis and mood of the moment. So therefore those saying that it is archaic is utterly nonsense. However, 天 (sky, heaven) and 日 (sun) are not interchangeable. 日落 = sunset, 天落 = something fell from the sky. 天子 = son of heaven = emperor is definitely not the same as 日子 = "days". 这天 = this day, NOT that day or someday. "Someday" = 有一天.