Leviticus 25:8

שבתות שנים [AND THOU SHALT NUMBER] SEVEN SABBATHS OF YEARS [UNTO THEE] — seven Shemittah-periods of years (cf. Onkelos). One might think that one may keep seven successive years as “Shemittah” and keep the year immediately following them as the “Jubilee”, Scripture therefore states “seven years, seven times”, consequently you are bound to say that each of these seven years of release has to be kept in its proper time (i, e. at the end of six years of work) (Sifra, Behar, Section 1 1). 'והיו לך ימי שבע וגו‎‎ AND THE PERIOD OF SEVEN [SABBATHS OF YEARS] SHALL BE UNTO THEE [FORTY AND NINE YEARS] — This mention of the sum total tells you that even if you have not kept the years of “Shemittah” keep the “Jubilee” at the end of forty-nine years (Sifra, Behar, Section 1 2). (The translation according to this Midrashic explanation therefore is: When seven Sabbaths of years (i. e. the seven seven-year periods) will be unto thee forty-nine years, thou shalt cause the horn to be blown etc. — no matter whether you have kept the intervening שמיטות or not.) The plain sense of the verse, however, is: The sum total of the periods of which each ends in a “Shemittah” (or, the sum total of these Shemittah-periods) will give thee forty-nine years.

And thou shalt number seven sabbaths of years to thee, seven times seven years; and the space of the seven sabbaths of years shall be to thee forty nine years.

וְסָפַרְתָּ֣ לְךָ֗ שֶׁ֚בַע שַׁבְּתֹ֣ת שָׁנִ֔ים שֶׁ֥בַע שָׁנִ֖ים שֶׁ֣בַע פְּעָמִ֑ים וְהָי֣וּ לְךָ֗ יְמֵי֙ שֶׁ֚בַע שַׁבְּתֹ֣ת הַשָּׁנִ֔ים תֵּ֥שַׁע וְאַרְבָּעִ֖ים שָׁנָֽה׃