Corpus Christi (Latin for Body of Christ) is a feast of varying importance in Western Christianity. For Roman Catholics it is of high importance.
Corpus Christi is always held on the Thursday after Trinity Sunday, or sixty days after Easter. As such, it is a mirror to Maundy Thursday, the Thursday next before Easter.
Corpus Christi is a thanksgiving for the sacrament of the Eucharist, and any celebration of it involves a special Eucharist.