On Yom Kippur, Jews all over the world partake of a ~25 hour fast. From 18 mins before sundown on Erev Yom Kippur (Erev is the Hebrew for Eve) to 3 stars in the sky the next night, the fast requires "nothing to pass your lips."
This means no eating, which everyone expects, but it also means no drinking. Not even water. The really hardcore do not shower (or even wash past their knuckles on their hands), do not brush their teeth (they do it after the pre-fast meal, but not the next morning when they wake up), do not take medications (unless a matter of life and death- the one exception to all Jewish laws is to preserve the sanctity of life), do not use lotion or chapstick or any salves/creams/balms of any kind, all prohibitions for the fast day.
Yeah, it's intense. I follow the majority of the rules (especially the ones not to eat or drink, I'm a little more lax on the showering, tooth brushing, and the lotions some years) and have ever since I was 13. The very orthodox fast while pregnant, while nursing, while moderately ill, while elderly, etc. The fast of Yom Kippur is considered so important to the observance of the holiday that you should do it above all other things- it is preferable to stay in bed all day fasting than to break the fast but go to synagogue.
To make the fast better, I follow these simple rules every year in preparation/day of:
1. Cut caffeine a week in advance.
2. Hydrate like it's your JOB all week long.
3. Pack in the food at dinner before the fast.
4. Take a nap!
Honestly I dread the fast every year because I really like to eat. But every year when the fast is over I feel like the anticipation was worse than the actual fast. For one, you're around fellow fast-ers all day, and misery loves company. For another, well, I mean it's just one day. They've been fasting on this one day for literally thousands of years. I know a lot of (non-Jews) think the no water thing sounds dangerous, but it's just one day. Honestly, it sounds worse than it is.
And, I don't mean to sound ridiculous, but there is kind of a new spiritual plane attained. Yeah, that sounds ridiculous. But you can maybe imagine, in hour 24 of the fast, you're just kind of over it. You're not really hungry anymore, you have gotten used to being thirsty. You're tired and a teeny bit loopy... and you're standing there (the last hour or so of services is all standing although you can sit for parts if you're too weak) and you're praying literally for your life and... it's just kind of awesome, in a weird way that makes it sound like I joined a cult.
The rabbi said one year that we fast because Yom Kippur is about honoring the needs of your soul and ignoring the needs of your body. And for a few hours in the middle of the day, it's hard to forget the needs of your body. The need for a drink (especially of coffee/caffeine of choice), the need for a snack, the need to sit down. But then, the needs of your body fade away and you can almost visualize yourself as pure soul, slipped the surly bonds of earth, having a chat with God.
The final service, Neilah, means closing. We traditionally think of it as the gates of heaven closing, that God opens the gates to listen for the day, but closes them as the last service ends. Alternately, we think of it as the book closing; the book of life is sealed as Yom Kippur ends and you're in or your out. This year the Rabbi suggested we think of it a different way; yes the door is closing, but we're on the inside- we're getting a closed door, private session with God. I like that idea.
Then its over. The shofar is blown, a quick evening service is conducted, and then you dig in. People hit the buffet, a cup of water (or two, or 10) is chugged, and people go back to talking and joking and eating and being normal. But for that one day, or at least for that one hour, you were special. You were pure soul.