The point is not to become an expert in a weekend or even a month-long class, but tobecome familiar and more comfortable with the language. The best time to find an immersion class is during the summer, since this is when most people have larger blocks of time to spare. Occasionally there are classes available at other times, particularly in April, December, and January. Ostaig offers a week-long class during April, for instance. One thing you can do about the limitation on dates is find a group of people who want to go at the same time.
Many schools are willing to host a group class at any time during the year.
If you are desperately seeking other students, try any Celtic mailing lists or Internet sites for fellow-minded folks, as well as any offline groups you know of. Just make sure you contact the school before making any sort of commitment, as these things are entirely voluntary on the school's part. Not surprisingly, most of the courses are offered in the Celtic origin countries. But an impressive number are offered in the US, particularly in the form of immersion weekends. These are almost always in the bigger cities on the east and west coasts, since it is usually Celtic organizations which sponsor them.