By way of contrast, the rules of the US Copyright Office state that a ~copyright~ registration is effective from the date the complete application is received in their mail room since it typically takes several months to process the application. The USPTO has no such rule, however, and a trademark registration is effective on the date that the USPTO issues the certificate. But remember that trademark rights accrue through use, not through registration. Thus, you have enforceable common-law trademark rights when you first make use of the mark in commerce. Registration and use are different concepts in trademark law. The only variance is when you file an Intent to Use federal trademark registration application, you do get a first-use date that “relates back” to the date you filed your application, so there is a benefit to filing an ITU application to preserve that first-use date.
The effective date of trademark registration is if and when your TM gets registered, and your application, if you're using your TM, will have 2 dates, the date of 1st use in interstate commerce, and date of 1st use anywhere (that's where foreign use can get mentioned, but since it's not in interstate commerce, it doesn't matter to the USPTO for a U.S. TM).
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
The effective date of registration is the date the registration certificate issues, not the date that you filed your trademark application. This is true in an application based on a foreign registration.
This is much less important than you might believe. Trademark protection is predicated based on the common law and statute (state and federal). You can enjoy trademark protection even before you apply for and receive trademark registration from the United States Patent and Trademark Office ("PTO"). If you can prove that you used the mark in commerce for a substantial period of time before the registration and that your mark acquired "secondary meaning" in the minds of consumers---i,e,m that it ws readily associated with your company's products or services, then you might be able to enforce your trademark against infringers even before achieving PTO registration.
Note, however, that registration with the PTO has many advantages. Such registration is national in scope, whereas common law trademark protection may be limited to the geographic locations in which your business operated. Further, registration with the PTO allows you to bring suit under federal trademark laws in Federal Court, and such laws provide a somewhat easier path to follow if you are attempting to prove infringement.
My guess is that you are asking this question because you are concerned about conduct of a competitor or infringer that occurred before you obtained your U.S. PTO registration. If so, then I urge you to retain counsel to represent and advise you. This is a complex and sophisticated area of law, and it would be a mistake to rely on the general advice provided by attorneys on this web-site with regard to business and legal decisions that you may need to make.