Testing, testing and more testing... and when you're done, do some more testing. The testing phase can take a while, depending on the complexity of the website and can be frustrating. Everything gets tested: functionalities and website conformity with generally agreed principles and standards; compatibility with all agreed browsers or the way in which the website functions on different devices; the functioning of the security system; etc.
Once this phase is done, we celebrate with a good meal and wine. Maybe even a night out [smiley face].