Per the IRS pub 501 :
Residency Test
To meet this test, your child must have lived
with you for more than half the year. There are
exceptions for temporary absences, children
who were born or died during the year, kidnapped
children, and children of divorced or separated
parents.
Temporary absences. Your child is considered
to have lived with you during periods of
time when one of you, or both, are temporarily
absent due to special circumstances such as:
• Illness,
• Education,
• Business,
• Vacation,
Military service, or
Detention in a juvenile facility.
