One of the Things that my Cub Scout Pack does every year is to plan out our calendar of events, in an annual planning calendar.
We go and get a copy of the local School System's calendar, community calendars, for us the Schedule of Nascar Races at the local speedway is important, because so many families have jobs either in the racing or hospitality industry that will be impacted on those weekends.
If your scout group encompasses people of several faiths, when you plan these things out, be sure to check for major religious events among those various faiths when planning.
One resource I found was the Interfaith Calendar which has religious observances from multiple faiths around the world. It's a great starting point, and note that the items in Bold type are generally the most important dates, although even minor ones may be mentioned.