More often than not, Catholic publications make the mistake of calling all celebrations a "feast". I just kept seeing this recently three times in a row: on Pentecost, on the Most Holy Trinity, and on Corpus Christi. The three of them are solemnities, not feasts, as magazines, and parish bulletins, websites and Facebook pages alike wrongly announced them.
In the liturgical calendar, celebrations have three degrees of importance: solemnities, feasts and memorials.
SOLEMNITIES are the celebrations of greatest importance. They begin on the prior evening with first vespers (evening prayer) and several of the solemnities have their own Vigil Mass. Both the Gloria and the Creed are recited. If a solemnity falls on a Friday in Lent, the observance of meat abstinence is not applied.
FEASTS are of second importance and are celebrated on a particular day. These do not have a first vespers or Vigil Mass the prior evening. An exception would be the feasts of the Lord which occur on Sundays in Ordinary Time and Sundays in the Christmas season. On these days, the Gloria is recited but not the Creed.
MEMORIALS have the lowest importance, and are classified as either obligatory or optional. Memorials commemorate a saint or saints. Obligatory memorials must be observed whereas optional memorials do not have to be observed.
EXCEPTION: There is an exception in which feasts and memorials become a solemnity: If they belong to the patron saint or mystery of faith of a parish. In those cases, at a parochial level, they are celebrated liturgically and treated as solemnities (including skipping the meat abstinence on a Friday in lent).