In calendar method of contraception, first day of fertile period is?

Shortest period gives information about the first day of the fertile period. The shortest cycle minus 18 days gives first day of fertile period. For example, in a cycle of 28 days (most common), first day of fertile period will be 10th day of cycle (28 —18 = 10). Longest period gives information about the last day of the fertile period. The longest cycle minus 10 days gives the last day of fertile period. For example, in a cycle of 28 days, last day of fertile period will be 18th day of cycle (28 —10 = 18).