Who knows fifty-three?
Please cite/link your sources, if possible. At some point in the next few days, I will:
Upvote all interesting answers.
Accept the best answer.
Go on to the next number.
53 is the maximum number of consecutive days with something "extra" in the evening prayers. And it happens twice. At least on this year's calendar, outside of Israel.
In the late summer / early fall: The night going into the 30th of Av gets Yaaleh v'yavo; the 29 nights of Elul all get L'David Hashem Ori V'Yishi (Psalm 27). The 22 nights of Tishrei through Shmini Atzeres get Psalm 27 as well. And the night concluding Shmini Atzeres gets "ata hareisa" and Simchas Torah. That makes 53.
On this year's calendar, the night concluding Simchas Torah gets no "ata chonantanu", as it's a Friday night. Just the usual, unmodified, Friday-night prayers.
In the spring / early summer: The night going into the 15th of Nisan is a holiday and has ya'aleh v'yavo; the next 49 nights have Sefiras HaOmer. The next two nights are holiday, and the next night concludes with "ata chonantanu." 53.
53 are the verses in Tzefaniah.
53 is the most number of Shabbatot needed to read all the weekly Torah portions based on the annual cycle.
There are 54 parshiot in total. The last parsha, Vezot Habracha is always read on Simchat Torah which never occurs on Shabbat in Disapora. (In Israel, Simchat Torah / SHemini Atzeret is same day, and Vezot could be read on Shabbat. However, in this situation, even Israel needs 53 weeks.)
For the 53 other parshiot, they would be read during a year that has 55 weeks. 2 of them are Hol Hamo'ed Pesach & Succot, so no weekly parsha is read, then. That leaves the remaining 53 weeks for the 53 parshiot.
This occurs during a leap year when the 1st day of Pesach is on Sunday and the 1st day of Rosh Hashannah was on Thursday. In such years, no parsha is doubled with another.