The Dental School application cycle begins on the first Thursday of June every year and ends at different times for each school (often around March of the following year). All applicants must use the American Dental Education Association (ADEA) sponsored “Associated American Dental Schools Application Service”, more commonly known as AADSAS Application. While AADSAS accepts applications into December/January, it is STRONGLY RECOMMENDED that applicants submit their information in June/July, a full year before the August/September in which you plan to matriculate to dental school. Applying early will greatly increase your chances of being invited for an interview and, ultimately, offered an seat at dental schools since admissions offices review applications in the order that they are submitted through AADSAS.
A general timetable for the dental school application cycle can be found here, courtesy of the Duke University Office of Health Professions Advising.