BUT THERE IS NOWHERE MENTIONED JANUARY OR DECEMBER $,$ NOTHING LIKE THAT?

Question

Three people must be on duty at all times. On what dates could lan start a $3$ day

vacation?

Calendar

SUN MON

TUE

WED

THU

FRI

SAT

$1$

$2$

$3$

$4$

$5$

$6$

$7$

$8$

$9$