Actually the Sate of Israel also uses both calendars. Our holidays here are the Jewish ones, except that some restaurants and hotels make a big deal out of January first and the night before for parties and all. Any excuse to make an "event." Remember that the Jewish New Year is Rosh Hashannah, a very serious time, frequently spent in synagogue being shaken to the soul by the sound of the shofar. It's a time of reflection, repentance (teshuva) that lasts a week and a half, until Yom Kippur. The
How did I get onto this? Back to the topic...
In Israel, using both calendars can be rather problematic. The school year begins on September 1, but when Rosh Hashannah will be "too early" school starts early, so the kids can learn something before all those Jewish Holiday and school vacations begin.
It was a nice way to start "birthday week."