Application date is the date on which the patent office received the patent application.
Publication date is the date on which the patent application is published (i.e. the information is available to public). This normally occurs 18 months after the priority date.
Priority date is the first date of filing of a patent application, anywhere in the world (normally in the applicant’s domestic patent office), to protect an invention. The priority date is used to determine the novelty of the invention, which implies that it is an important concept in patent procedures. For statistical purposes, the priority date is the closest date to the date of invention.