他日 is a literary/formal phrase meaning 'some other day' or 'later on'. It appears in written Chinese (classical-style prose, news commentary, speeches) rather than everyday conversation. In daily speech, 改天 (gǎitiān) or 以后 (yǐhòu) would be used instead. 他日 often carries a slightly elevated or solemn tone, and is typically used to refer to a future time that is not precisely specified.