The probability of randomly selecte one day as Sunday is 1/7, and that probability of randomly selecting two day as Sunday is1/7 *1/7 =1/49; Take two days at random, the probability that the first day is Sunday and the second day is not Sunday is1/7 * 6/7 = 6/49;

Randomly take two days, and the probability that the second day is the first day of Sunday instead of Sunday is1/7 * 6/7 = 6/49;

The above two situations add up to 6/49+6/49= 12/49.