Rashi on Leviticus 25:8/רש"י על ויקרא כ״ה:ח׳

1שבתות 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).

2'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.