However, it seems to me that today is entirely the wrong day to to this on. You see, at the time they were using the Julian calendar, and it was the first day of July, 1690. In the Gregorian calendar at that point in use in mainly Catholic dominated areas of Europe it was the 11th July.
10 years later, in 1700 (or arguably 1699) the Julian Calendar contained a 29th February that didn't exist in the Gregorian Calendar. (To be really picky it included an additional "sixth day before the calends of March" meaning that the last 5 days of the month had a different name, and that the 24th was a day that didn't exist in the Gregorian Calendar; but lets not be really picky).
52 further years later, Great Britain and Dominions moved to the Gregorian Calendar, by decreeing that 14 Sep followed 2 Sep. It would appear that at this time the Orangemen moved the celebration of the Battle of the Boyne to the 12th July.
For the rest of that century this will have provided the correct Julian date for the celebration, but in 1800 (or arguably 1799) the Julian Calendar once again included an additional "sixth day before the calends of March" which the Gregorian Calendar didn't include; such that at this point the Orangemen were one day out and should've been celebrating on the 13th. This happened again in 1900 (or 1899), and will occur again in 2100.
So, in conclusion, the logical dates to celebrate the Battle of the Boyne would be:
- 1st July: Using the standard "sod it" approach to dealing with these renumbered days.
- 11th July: The Gregorian date upon which the Battle actually occurred.
- 14th July: The actual 1st of July in the Julian Calendar.