As on Shabbat, don't fast on Jewish festivals (except Yom Kippur!) past halachic midday.

• If you will not finish shacharit before halachic midday, you should eat or drink earlier in the day, even before you begin shacharit—water can be sufficient for this purpose.
• If you will finish shacharit, but not musaf, by halachic midday:
  • Finish shacharit,
  • Make kiddush,
  • Eat some mezonot, and then
  • Return to say musaf.

We are required to eat two meals on Jewish festivals, each preceded by kiddush, one in the evening and one in the morning. For both meals for each Jewish festival day (and all three meals on Shabbat), say ha'motzi over two complete loaves of bread, each of which is at least 1.3 fl. oz. in volume. 

Source of Saying Jewish Festival Kiddush
Some kiddushes are commanded by the Torah (d'oraita); the others are from Chazal (d'rabanan), as follows:
Kiddush d'Oraita

• First night of Jewish festivals
• Night of seventh day of Passover
• Night of Shmini Atzeret.

Kiddush d'Rabanan

• First day of Jewish festival
• Second night of Jewish festival
• Second day of Jewish festival
• Seventh day of Passover
• Eighth night of Passover
• Eighth day of Passover
• Shmini Atzeret day
• Night and day of Simchat Torah.

Source of Jewish Festival Kiddush Location
Saying Jewish festival kiddush at the place where you will eat your meal is a rabbinical (d'rabanan) enactment.
Jewish Festival Kiddush-Meal Quantities: Evening

• For evening kiddush for a first-night (d'oraita) Jewish festival, a minimum of 4 fl. oz. (119 ml) of wine must be blessed on and at least half must be drunk.
• For evening kiddush for a second-night (d'rabanan) Jewish festival, a minimum of 3.3 fl. oz. (99 ml) of wine must be blessed on and at least 2.0 fl. oz. must be drunk.
• For the evening meal, as on the first two Shabbat meals and for all required Jewish festival meals, a minimum of 1.9 fl. oz. (56 ml) of bread must be eaten within four minutes.

Eating Requirements for Jewish Festival Morning Kiddush
There are two separate eating requirements during the daytime. They may be combined (say/hear kiddush and start the main meal right away) but are often done separately (say/hear kiddush and then eat some light foods and beverages; the main meal is eaten later in the day).
Note Since eating and drinking requirements on all morning kiddushes (both Shabbat and Jewish festivals) are d'rabanan, the required beverage amount for morning kiddush is only 3.3 fl. oz. (99 ml) instead of the d'oraita 4 fl. oz. (119 ml) (which is required for kiddushes for Shabbat evening and all first-night Jewish festivals).

• Morning kiddush requires a halachically legal “meal” with these elements:
  • Blessing on a minimum of 3.3 fl. oz. (99 ml) of wine (or other beverage),
  • Someone's drinking at least half the beverage, followed by
  • Eating at least 1.3 fl. oz. (39 ml, or 1/6 cup) of some type of mezonot within four minutes. 
    Note If you drink at least 3.3 fl. oz. (99 ml) of wine within 30 seconds, you do not need to eat mezonot.
    Note You do not need to drink the wine or other kiddush beverage to fulfill “establishing a meal.” You may hear kiddush and then simply eat the required amount of bread or mezonot. This applies to Shabbat or Jewish festivals, evening or morning.
     

 

• The real meal (kovei'a se'uda) requires eating at least 1.9 fl. oz. (56 ml) of bread (or matza during Passover!) within four minutes. It must include enough food to satiate. 
  Note You can simultaneously fulfill the requirement to “establish a meal” and to “eat a meal” by eating one (the same) piece of bread.

Do not eat a full meal (meaning, do not eat bread or a lot of mezonot) after halachic midday on the afternoon before a Jewish festival.

Shiv'a that is interrupted by a Jewish festival does not resume after being interrupted.

As on Shabbat, once you have said the amida of Jewish festival shacharit, you may not eat any food until you have said (or heard) kiddush and finished kiddush requirements by either drinking 4 fl. oz. (119 ml) of wine/grape juice or eating at least 1.3 fl. oz. (39 ml, or 1/6 cup) of mezonot or bread.

As on Shabbat, you may eat non-mezonot and non-bread food before praying Jewish festival shacharit and without making kiddush, in order to avoid hunger or hypoglycemia (low blood sugar).

See Eating from Start of Jewish Festival until Kiddush.
 

For women and minimum prayer before eating prior to Jewish festival kiddush, see Women and Minimum Prayer before Jewish Festival Kiddush.
 

You must eat a second meal on Jewish festival (or Shabbat) day with 1.9 fl. oz., or 56 ml, of bread--even if you already said ha'motzi and ate bread at kiddush.

Once the Jewish festival begins for you—either at sunset or before (such as if you lit Jewish festival candles)--you may not eat or drink before hearing kiddush.  Women and girls may make kiddush soon after lighting candles.

A woman who has already lit Jewish festival candles may not say mincha for the afternoon before a Jewish festival, even if she lit (after plag ha'mincha) long before sunset time, unless she intended not to begin the Jewish festival when she was lighting the candles (and intending to begin later should only be done in urgent situations, not routinely.)

The minimum prayer that a woman should say on Jewish festival (or Shabbat) morning before saying kiddush and eating some food is birchot ha'shachar.
 

See Appetite for Jewish Festival Dinner.

If you are late to minyan on Shabbat (or Jewish festival) morning, say:

• Shacharit while the minyan is saying the musaf amida (as long as it is not yet too late to say shacharit), and then say
• Musaf while the reader is repeating the amida.