Use ～をもって in formal writing and announcements to mark either the means by which something is done (「書面をもって通知する」) or the point in time at which a state takes effect or ends (「本日をもって」). It is a stiff, documentary register — at home in official notices, contracts, ceremonies and formal email, but odd in casual speech, where で or から/まで do the same job. The time usage almost always attaches to endings, deadlines and effective dates.