What type of employment visa? E.g. if your employer applied under H1-b visa regulations, H1-b visas are issued with validity dates starting 1 October. You could be getting your visa (IF it is H1-b, and IF it is approved) this week, and you can enter US on 1 October or thereafter.
Some other type of visa? IF approved & issued, the visa will tell you the validity dates, and you cannot enter US until that visa is valid. E.g. if the visa becomes valid, say 15 November, you cannot enter US before 15 November. With work visas, you report to your employer immediately upon arrival to start work.