Availability to Work

Fei

Cannot work Sundays, Saturdays

George

Cannot work even numbered Fridays

Helen

Cannot work odd numbered Wednesdays

lan

Must always work Mondays

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

$4$

$5$

$11$

$12$

$18$

$19$

$25$

$6$

$7$

$8$

$14$

$15$

$21$

$1$

$2$

$9$

$13$

$20$

$27$

$28$

$29$

$30$