The way of use is contingent on the type of contraceptive, i.e. mono-phase, phase, or daily use. The 21-day tablets are taken for 21 day, then an interval of 7 days is made and the use is restarted. The 28-day pills are used daily without interruptions as they have 7 empty pills which when you use them basically mean a break from hormones use.
It is ok to begin the use of the tablets on any day but there are special rules for females who had a recent abortion, miscarriage or recent labor.
Besides, contingent on your cycle length and the day of the first tablet intake, you might have to use supplementary contraception means for the first week of tablets use.