For a week of a certain month, if the date numbers of its seven days are divided by 7, and the seven remainders obtained are added up, the number is exactly equal to the date number of the first day of the week. What is the first day of the week?
某月的一周，如果把它七天的日期号数用7去除，将所得的七个余数相加，得数正好等于这周第一天日期的号数．这一周的第一天是多少号．
<gadget id="calculator">0 + 1 + 2 + 3 + 4 + 5 + 6</gadget>
<output>21</output>

<result>21</result>