When planning an event you always start with the 5 Ws and 1 H:

• Who – is the host and who are the guests?
• What – category/type of event is this?
• When – will this be held?
• Where – will this be held?
• Why – is the host having this event? 
• How – will you execute the event? 

The Who and the What is easy enough (read here) but when you reach When that’s when it gets tough before you can actually book any vendor.
So how do you choose a date for your event? 

• Consider the season – If you plan to have an outdoor event, make sure that it’s not during the rainy season. Other seasonal factors that can impact your event attendance would be holidays, sports schedules (remember the Pacquiao days?), weather, and school calendar.
• Avoid dates with high competition – Chinese couples have their weddings on auspicious dates. Trying to find an available church can be a challenge if there will be a lot of Chinese couples who would like to get married on the same day and at the same time at the same church as you. Corporate or family-oriented events are normally held during weekends wherein you expect a lot of people to be free to attend your event. Make sure you book ahead of time to get the venue you want especially if the venue is very popular for events. 
• Factor in Public events or Holidays –  Unless your event has something to do with the holiday itself or a public event, it would be hard to get people to attend your own gathering. Holidays that fall on a Friday mean a long weekend for employees and we all know that people love long weekends. So scheduling a business or work-related activity/event that does not bring in the fun will not be appealing to your employees. They would rather go out of town and spend time with their family. Elections, festivals, and other similar events are dates that you have to avoid.
• Know the favorable days and times of your guests – Consider the “prime days or times” your guests are most likely available to attend your event. If your activity would target consumers or if you’re having a social event, evenings and weekends are the best choices. If this is business, related to seminars or conferences, any day between Wednesday to Friday is best. 

Try to find out more about your target audience/attendees and work around their schedule. Once you have your date, looking for a venue comes next.