Excellent, nice to don one's finery to go meet the royals!
Ann and I were invited to one of Prince Charles' garden parties at Holyrood House, a few years back and most of the men wore kilts, though surprisingly not the Prince on that day.
after all I'm not sure I'll ever get a second
They say that if you should ever receive a second invite to a royal garden party it is etiquette to decline the invitation as the objective of these events is for the royals to meet as many of their subjects as possible rather than repeatedly meeting the same subjects.