We aimed to explore whether a single-day of fasting (SDF) increase emergency room (ER) visits due to renal colic (RC). We elected to concentrate on Yom-Kippur (i.e.: SDF), the holiest day in Judaism. Food and liquid consumption is prohibited during this day for 25 h, and an estimated 50-70% fasting rate is observed. SDF always takes place between mid-September and mid-October during which the temperature in the Middle-East ranges between 19 and 30 degrees C. ER visits for RC between 01/2012 and 11/2019 were reviewed, and the Gregorian days on which SDF occurred were retrieved. The number of ER visits for RC was compared between SDF and the surrounding days/months as well as to another single-day "standard" holiday (SDSH) that precedes SDF in 10 days and is not associated with fasting. Of 11,717 ER visits for RC, 8775 (74.9%) were males. Male:Female ratio was 3:1. The mean daily number of ER visits for RC during the 3 days following SDF was 6.66 +/- 2.49, significantly higher compared with the mean annual daily visits (4.1 +/- 2.27, p < 0.001), the mean daily visits during the week prior to SDF (5.27 +/- 2.656, p = 0.032), and the mean daily visits during September (5.06 +/- 2.659, p = 0.005), and October (4.78 +/- 2.23, p < 0.001). The mean number of ER daily visits for RC during the 3 days following SDSH, 5.79 +/- 2.84, did not differ compared with the mean daily visits during September and October (p = 0.207; p = 0.13, respectively). It was lower compared to SDF, however statistically insignificant (p = 0.285). A single-day fasting may increase ER visits for RC. The mechanism underlying this phenomenon is unknown.