Thanks in part to the Common Application, schools are receiving more applications than ever. There are simply not enough spots at the most competitive and popular schools to accommodate qualified students. Remember, many students are rejected even though they are bright and capable of doing the work. (So try not to take it personally!) The more students who apply, the higher the GPA of accepted students will be. And of course, schools are also looking for diversity. So if 50 kids from your high school apply, they won't all get it in because the school wants to also accept students from the high school across town, or across the country, or across the ocean. You get the idea.
This system of mid-year admittance allows schools to have "back up" students to replace upper classmen who are spending a semester abroad or graduating early. And some freshmen won't return for a second semester due to financial, academic, or personal reasons. Schools need to fill these spots in order to sustain themselves financially.
So, what to do you when you receive an acceptance letter ... but the fine print says "not until January?" The advantages of accepting this offer include the opportunity to attend a school where you may have not otherwise been accepted. The school may offer an opportunity to study abroad or work as an intern fall semester. Or, you could find opportunities for yourself volunteering (at a local school, hospital, or animal shelter, for example). This opportunity to pursue your interests may help you choose a major and attack your studies with purpose. Some students appreciate college all the more if they take a break after high school, especially if they're feeling burnt out.
If you do opt to start mid year you should know that it could be difficult to make connections with other students, since you might not be in the freshman dorm or perhaps you'll miss out on traditional fall orientation activities. This is something to consider if you're on the shy side. Also, keep in mind that you may not be able to take the classes necessary to graduate on time. How important is it to you to graduate at the same time as your friends? Are your parents willing and able to pay for an addition semester or year of college?
There's no right answer here. It all depends on what you would do with the time off, how you feel about not doing what most of your friends are doing, and how badly you want to attend the school.
I believe that there are many excellent schools (including some you may not have heard of) where you can have a great college experience. Be open minded and do what's right for you.