Why is the ninth month called September?
late 13c., "last will disposing of property," from Latin testamentum "a will, publication of a will," from testari "make a will, be witness to," from testis "witness," from PIE *tris- "three" (see three) on the notion of "third person, disinterested witness."
Use in reference to the two divisions of the Bible (c.1300) is from Late Latin vetus testamentum and novum testamentum, loan-translations of Greek palaia diatheke and kaine diatheke. Late Latin testamentum in this case was a mistranslation of Greek diatheke, which meant both "covenant, dispensation" and "will, testament," and was used in the former sense in the account of the Last Supper (see testimony) but subsequently was interpreted as Christ's "last will."
occurs twelve times in the New Testament (Heb. 9:15, etc.) as the rendering of the Gr. diatheke, which is twenty times rendered "covenant" in the Authorized Version, and always so in the Revised Version. The Vulgate translates incorrectly by testamentum, whence the names "Old" and "New Testament," by which we now designate the two sections into which the Bible is divided. (See BIBLE.)