Yom Kippur, the Jewish Day of Atonement, falls eight days after Rosh Hashanah, the Jewish New Year. Most notably, the day is observed by living without many humanly pleasures: Observant Jews are not supposed to wear leather or makeup and, most importantly, they are not supposed to eat.
The fast is one of many that occurs in the Jewish calendar year —Tisha B'av, a day of mourning a few months earlier is also a fast day — when observers are instructed to deprive themselves of food and drink from sundown on the eve of the holiday to the sunset closing out the holiday.
While food and drink are forbidden on Yom Kippur, is plain old water okay?
Like everything in Judaism, the ritual observance is up for interpretation and mandated differently by various sects and religious leaders. The elderly, ill or those who are pregnant are instructed not to fast at all, but for those healthy enough to spend the 25-hour fast without food or drink, strict rules do apply.
According to Chabad, a sect of Orthodox Judaism, all eating and drinking is forbidden on Yom Kippur — water included. Many fasting guides recommend drinking extra water in the days leading up to Yom Kippur, to hydrate one's body in preparation for the water-free fast. Many take this water ban to the extreme, abiding by the rules not to even swallow necessary medications with water and not to use water or mouthwash to even rinse your mouth if it feels dry or uncomfortable.
Reform Judaism, however, takes a more liberal approach when it comes to water on Yom Kippur. While the sect does not encourage the drinking of water while fasting, ReformJudaism.org suggests that "during the fast, if you get dizzy or lightheaded try sitting down for a little while. If the feeling persists, or if you have other worrying symptoms, please drink some water immediately and eat a small amount of food. Judaism does not condone endangering your life in order to fast, even on Yom Kippur."
If it's a matter or life or death, or serious health concerns, water is indeed permissible on Yom Kippur. If it's just a comfort thing or desire for an ice cold LaCroix, fasters will have to wait until sundown. But how good will that first sip of water taste after 25 hours without?!