A Kiddush cup is called a “Kos Shel Brochoh”, i.e. a cup with wine that we bless over.
There are many occasions that we bless over wine, not just Kiddush. Although we make Kiddush twice on Shabbat and festival days, morning and evening, we also use a Kiddush cup when Shabbat or festivals end, to make Havdalah and to differentiate between Shabbat and the rest of the week.
In addition, we use Kiddush cups four times on Seder nights, twice under the Chuppah, and for a week after that, for Grace After Meals. We also name baby boys at their Brit Milah over a cup of wine, and when we say Grace after Meals with other people on occasion.
Blessings over wine are made together with other blessings appropriate to each occasion and making them over a cup of wine lends importance to each occasion. However, the main obligation to bless over wine is on Friday nights, as we are commanded (Exodus 20:7) to “Remember the Sabbath day by sanctifying it.” Hence the requirement to make Kiddush on Shabbat.
A “Kos Shel Brochoh” must be clean and unflawed, and so should be washed if somebody else has used it, a point often overlooked! In addition, because of its importance, people often look for beautiful cups to make Kiddush over, which is why Judaica items are generally so beautiful and ornate. We show respect for the Mitzvot that we use them for. A beautiful cup for a beautiful Mitzvah!
Wine must also be sufficiently diluted with water to be drinkable, but not too diluted. The Talmud required at least 25% of the liquid to be wine, as a greater proportion would have been difficult to drink, at least in those days. Nowadays, wine has already been diluted by the winery, and adding any more water would often render it water rather than wine. We should be able to taste and appreciate the wine that we drink.
If somebody can’t drink wine, for health or other reasons, then we can use other drinks in most cases, although Friday night Kiddush is a little more difficult. Acceptable alternatives to wine have to either be alcoholic liquids, according to some Rabbis, whereas some authorities opine that if a liquid, such as fruit juices or coffee, is considered important and suitable for guests in a country where it is being used then this is also acceptable.
A Kiddush cup has to hold at least 75-86 milliliters, (2.5-2.9 fluid ounces) to make Kiddush over, although wine given out to other people can be in much smaller amounts or cups.
We generally completely fill a cup before making Kiddush, if possible. If we don’t have enough or can’t drink that much wine, then there should be enough wine in the cup to drink normally.
Wine must also be unused. If it has had Kiddush made over it or been tasted, then fresh wine has to be added to it. Otherwise, it is not fit to make a blessing over a Mitzvah with it. Nevertheless, it can be drunk and blessed over if it is only being used to drink from, and not for a specific Mitzvah.
People making a blessing over a Kos Shel Brochoh should be dressed appropriately and have something on their head.
Ideally, when we are about to start Kiddush or some other Mitzvah, we should hold the cup with both hands, out of love for the Mitzvah we are about to do. We should then lift it about 4” up, (unless we are not strong enough to do so) and move the cup to our right hand (unless we are left-handed).
We should keep looking at the cup as we bless, as this is what we are blessing over. For the same reason, we should not discuss anything else besides Kiddush or any other Mitzvah that we are blessing over once we hold the cup, and the same applies to anybody listening to the blessing.
The person making Kiddush should drink his portion immediately, after which everybody else can drink, either from small cups served beforehand, or if the person making Kiddush gives cups out. In the latter case, his wife should be the first person served wine after whoever made Kiddush has drunk. He has to drink first because we must eat or drink immediately after having made a blessing over food or drink.