Unlike most of the world that celebrates Corpus Christi (latin for "Body of Christ") on Thursday because the Eucharist was instituted on Holy Thursday, the United States of America (in all but a few traditional chapels) celebrates it on the following Sunday.
While many traditional parishes (that will say the Traditional Latin Mass) will still celebrate Corpus Christi on the Thursday after Trinity Sunday , all parishes (even traditional ones) have permissions to transfer its celebration to the subsequent Sunday (i.e. 1st Sunday after Pentecost). This indult to the United States was granted by His Holiness Pope Leo XIII.
No such indult exists for the Ascension or for Epiphany dating back to the time of John XXIII or earlier.
For those praying the traditional Divine Office, keep in mind that the Office of Corpus Christi does not transfer. Corpus Christi remains the Thursday after Trinity Sunday.