A common year starting on Monday is any non-leap year (i.e. a year with 365 days) that begins on Monday, 1 January, and ends on Monday, 31 December. Its dominical letter hence is G. The most recent year of such kind was 2007 and the next one will be 2018 in the Gregorian calendar or, likewise, 2013 and 2019 in the obsolete Julian calendar, see below for more.
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Gregorian Calendar
In the (currently used) Gregorian calendar, the 15 types of years repeat in a 400-year cycle (20871 weeks). Forty-three common years per cycle or exactly 10.75% start on a Monday. The 28-year sub-cycle does only span across century years divisible by 400, e.g. 1600, 2000, and 2400.
Julian Calendar
In the now-obsolete Julian calendar, the 15 types of years repeat in a 28-year cycle (1461 weeks). Each leap-year dominical letter occurs exactly once and every common letter thrice.
The final two digits of Julian years repeat after 700 years, i.e. 25 cycles. When starting to count in 2001 for instance, every 1st, 7th and 18th year of these Julian cycles is a common year that starts on a Monday, i.e. ca. 10.71 % of all years. They are always 6 or 11 years apart.