Saturday
It may begin on any day
New Years Day 2000 was on a Saturday.
It was a Tuesday.
It depends where you live
Thursday
monday
If you mean what day of the week did July begin, the 1st of July 2013 was a Monday.
July 29, 2000 was the 211st day of the year 2000 in the Gregorian calendar. There were 155 days remaining until the end of this year. The day of the week was Saturday.
Saturday
7 January 2000 was a Friday.
20 January 2000 was a Thursday.
15 April 2000 was a Saturday.