As soon as a website launches, the new owners start to ask, "Is anyone looking at it?" and "What can I do to get more traffic?" Psychologically, it's time to make the change from building to marketing.
Or is it?
More likely, it's time to switch from building a site to building out a site. You probably have a good design but little content on your site on the day of launch, so it may not be the best time to introduce a ton of visitors.
Also, your site most likely hasn't been tested against real-world users, so there may be things that were overlooked in the design phase that you'd prefer to address before a ton of people visit.
In some cases, sites could start rolling out marketing efforts in stages as certain personas are handled well, or certain types of pages reach acceptible quality standards. For example, an online retailer could start testing pay per click ads for products as the pages reach a certain quality standard (assuming the checkout functionality is also ready for prime time).
For PR and other link building efforts, make sure you have an easy to understand, professional, offering that seems link-worthy. Clearly, day-1 after launch may not be the time when that standard is hit.