I am going to apply for a work permit soon. I already have a positive LOM. Let's say, I have a work permit already. When is the earliest date I am allowed to enter the country, provided my employment in Canada is supposed to start on July1? Thank you for your time and cooperation.

If you have a work permit already, then the earliest that you can enter Canada is right away. If you have an LMO and wish to enter prior to the stated start date, then you may seek to do so as a visitor.