Just wondering, whether that Monday or the following Sunday was an obligation in the US.
I’m in the Archdiocese of Minneapolis-St. Paul, if that helps.
Yes, the following Sunday is a day of obligation, as is every Sunday.
In the United States of America, in addition to all Sundays, the following days are obligatory:
Dec. 8 - Immaculate Conception of the BVM
Dec. 25 - Nativity of Our Lord
Thursday of the Sixth Week of Easter - Ascension of Our Lord (although most ecclesiastical provinces move this to Sunday; only the Provinces of Boston, Hartford, New York, Newark, Philadelphia, and Omaha keep the feast on the correct day as a day of obligation)
Additionally, the following days are obligatory if they fall on a Tuesday through Friday:
Jan. 1 - Mary, Mother of God
Aug. 15 - Assumption of the BVM
Nov. 1 - All Saints