Thursday
The most recent year with the same day-date correlation as 1980 was 2008. The next year with the same day-date correlation as 1980 will be 2036.
According to the online calculator, the year of 535 started on a Monday. During the year 535, John II ended his reign as the Catholic Pope and St. Agapitus I began his.
The calendar in the year 1120 started on a Thursday. As a leap year, the year 1120 included an extra day. The next leap year starting on a Thursday will be in the year 2032. The previous was in the year 2004.
Wednesday
Monday
It was a Tuesday.
Yes they do.
The Sabbath occurs every week.
Dates are written month/day/year in the US and day/month/year in most other countries. June 4 1980 was a Wednesday, and 6 April 1980 was a Sunday.
July 30th, 1980 fell on a Wednesday. The following year, it fell on a Thursday.
1980 was a leap year that started on a Tuesday, so March 4th was a Tuesday.
January 31st, 1980 fell on a Thursday. The following year, it fell on a Saturday.
According to the Gregorian calendar, the first day of the week of the year 480 "anno domini nostri iesu christi ", which has been a leap year, was a Monday.
January 1, 2009 fell on a Thursday.
It may begin on any day
It was a Sunday.
That was a Friday. See link to day-of-week calculator.