Result of search for "4521":
4315. prosabbaton pros-ab'-bat-on from 4253 and 4521; a fore-sabbath, i.e. the Sabbath-eve:--day before the sabbath. Compare 3904.
4520. sabbatismos sab-bat-is-mos' from a derivative of 4521; a "sabbatism", i.e. (figuratively) the repose of Christianity (as a type of heaven):--rest.
4521. sabbaton sab'-bat-on of Hebrew origin (7676); the Sabbath (i.e. Shabbath), or day of weekly repose from secular avocations (also the observance or institution itself); by extension, a se'nnight, i.e. the interval between two Sabbaths; likewise the plural in all the above applications:--sabbath (day), week.