No, there is no penalty for one month. There is a short gap allowed of 2 months.
Short coverage gap (code “B”).
You generally can claim a
coverage exemption for yourself or another member of your tax household for
each month of a gap in coverage of less than 3 consecutive months. If an
individual had more than one short coverage gap during the year, the individual
is exempt only for the month(s) in the first gap. If an individual had a gap of
3 months or more, the individual isn't exempt for any of those months.
For example, if an individual
had coverage for every month in the year except February and March, the
individual is exempt for those 2 months. However, if an individual had
cover-age for every month in the year except February, March, and April, the
individual isn't exempt for any of those months.
An individual is treated
as having coverage for any month in which he or she has coverage for at least 1
day of the month.