Check the dots' "Visible" property, you'll see they're based on the existence of an event at a date.
If you want to make the dots visible based on the selected record of your gallery, you can change the dots' "Visible" property to something like this, if you're using the exact same calendar Power Apps gave you as a sample:
Circle.Visible:
And(
Gallery.Selected.StartDate <= DateAdd(_firstDayInView;ThisItem.Value;TimeUnit.Days),
DateAdd(_firstDayInView;ThisItem.Value;TimeUnit.Days) <= Gallery.Selected.EndDate
)
Or, if you want to use all the records at once, you can calculate the earliest "StartDate" and the latest "EndDate" to get a range of date. The "And()" function uses the variables "_startDate" and "_endDate" instead of the gallery's item.
With(
{
_startDate: Min(Gallery.AllItems; StartDate);
_endDate: Max(Gallery.AllItems; EndDate)
};
And(...)
)