Not that I have a ton of experience with this, but from what I can observe, typically wedding dates are picked based on their significance or sentiment. The date could be the day the couple met, their first date, the first time they exchanged 'I Love Yous', their anniversary, their parents' anniversaries, their grandparents' anniversaries, they day they got engaged, and so on. The special value the date holds is extremely important to the couple, so much so that they would plan their whole wedding around this specific day.
Other couples pick by season, weather, or location. Will their wedding be indoors or outdoors? Do they want a summer wedding? Spring? Fall? Winter? Do they want to avoid a day where in the past it has snowed? Rained? They may also take into consideration holidays or other events that may be going on. Sometimes it's picked based on the availability of the venue they want to have the ceremony or reception at. Or if they're planning on have a destination wedding, there is much less flexibility in choosing a specific date.
Another huge factor now-a-days is price. There are so many budget-conscious brides that plan their weddings based around saving money. June, August, and September are the most popular times to marry, so prices are inevitably higher. But if, for example, you're planning a wedding in January, March, or December, it may cost less because not as many couples are waiting and offering more money for these dates. Days of the week also matter; Saturday nights carry a hefty price tag, but getting married during the week will give you the upper-hand in negotiating a reasonable price.
After taking all of this into consideration, it is beneficial to sit down and visualize what you want your wedding to look like before settling on a specific date. Create a list and focus on what you want your wedding day to be and how to portray that in a date. Most importantly, give yourself plenty of time; don't rush it! A lot of thought goes into devouring the details of your wedding date but it will be something to celebrate for many years to come. Best wishes to all our future brides and grooms!