It is almost impossible to choose a date that will suit everyone, all you can do is take as many factors into account as possible and choose the most convenient date for most people.
Avoid school holidays as you will lose half of the parents due to their family holidays and other commitments. If your company is located on the edge of a county, holidays may not be at the same time, so you need to check any relevant local authorities.
This causes ‘hot dates’ in the calendar. These are dates of maximum convenience and event companies can find themselves with three or four times the usual number of events in those periods. Companies are giving 3-4 weeks’ notice on events on average, but the sooner you can book the better.
The other thing to avoid is major sporting events. If you search online for a sporting event calendar you can avoid the big dates, for example major matches, Wimbledon and so on. Sometimes companies will arrange for a large screen to be set up so people can still watch the event but you do lose some of the focus.
By this stage you will have a selection of possible dates. You then need to work from the most senior attendees down, or those who it is most important attend. People do tend to look at who will be there and draw their conclusions about how important it is that they attend. Sometimes losing a key influencer is followed by others dropping out. This all needs to be considered.
Finally, check the availability of the hotel and activity provider. Some people take this as the first step but it's easier for the activity provider and hotel if you approach them with a selection of dates. Many hotels will not discuss prices unless you give them a date, not because they're being difficult but because availability is what everything hinges around.
Remember that the more notice you work with the better. Last minute bookings inevitably lead to compromise.