It's a famous question but I'd like hear all different answers. The pasuk says תספרו חמישים יום, yet we only really count 49. Why?
Rashi addresses this as follows (from the chabad website) on Vayikra 23:16
the fiftieth day, [on which] you will bring a meal offering to the Lord from the new [wheat crop]: [lit., “(You shall count) fifty days and bring a meal offering to the Lord from the new (wheat crop).” But we count only forty-nine days. Therefore, the meaning is:] On the fiftieth day, you shall bring this [meal offering of the new wheat crop]. But I say that this is a Midrashic explanation of the verse [since it requires the forced attachment of the words חֲמִשִּׁים יוֹם to the continuation of the verse regarding the meal offering, whereas the cantillation signs attach them to the preceding words regarding the counting]. But its simple meaning is: “until [but not inclusive of]…the day after [the completion of] the seventh week, which is the fiftieth day, shall you count.” Accordingly, this is a transposed verse.
The Rabbeinu Bahya summarizes this point as
ועל כן תמצא טעם במלת תספרו ואינה דבקה למלת חמשים, אבל חמשים דבק לוהקרבתם כי הספירה מ"ט והקרבן ביום חמשים
That the word "you shall count" isn't attached to the word "fifty" but to "and you shall bring an offering" because we count 49 and the sacrifice is on the fiftieth (my paraphrasing)