An account of the great battle of Otterburn, 1388 can be found on Wikipedia.
There appears to be some confusion as to whether the battle took place on the 5th or the 19th August 1388.
Jean Froissart was a medieval poet and court historian. He published his account of the “honourable adventures and feats of arms” during the period 1326 – 1400 in his Froissart Chronicles. These works are perceived by many as being of vital importance to informed understandings of European in the 14th century.
His account of the great battle can be found in volume III, Chapter CXLI, starting at page 648 of this translation. Froissart does not specify a date for the battle, but describes how the Scottish barons and knights met for a festival “on the borders of wildest Scotland, in a city called Aberdeen” and “There it was agreed that in the middle of August of the year 1388 they would all meet, each with his force of men, near the borders of Galloway (Froissart’s term for south – west Scotland) at a castle deep in the forests called Jedburgh”.
As this was in the middle of August and before the battle, this strongly points to the 19 August for the date of the battle and not the 5th August.
A more recent account is made by Robert White in his book “History of the battle of Otterburn fought in 1388”, published in 1857.
On page 59 White states “ Thus on the night of Wednesday the 19th and morning of Thursday 20th August, the latter being the time of the full moon, in the year of our lord 1388, was stricken the far-famed Battle of Otterburn.”
During this time, and until 1752, the Julian calendar was in use and the dates published in these accounts of the time will be the dates according to the Julian Calendar.
| New Moon | First Quarter | Full Moon | Last Quarter |
|---|---|---|---|
 |  |  |  |
| 10 January | 16 January | 24 January | 1 February |
| 8 February | 15 February | 22 February | 1 March |
| 9 March | 15 March | 23 March | 31 March |
| 7 April | 14 April | 22 April | 30 April |
| 6 May | 13 May | 21 May | 29 May |
| 4 June | 12 June | 20 June | 27 June |
| 4 July | 11 July | 19 July | 26 July |
| 2 August | 10 August | 18 August | 25 August |
| 1 September | 9 September | 16 September | 23 September |
| 30 September | 9 October | 16 October | 22 October |
| 30 October | 7 November | 14 November | 21 November |
| 29 November | 7 December | 13 December | 21 December |
Close examination of the lunar tables for 1388, reproduced above, using the Julian calendar, confirms that the 5th of August 1388 was 3 days after a new moon and there would have been little light that night for a battle.
There was a full moon on the 18th August 1388 therefore, if indeed the battle was fought “by the light of a harvest moon” it is probable that the battle took place on 19 August 1988 and not 5 August 1388.
Further evidence in support of this can be found in this extract from James Douglas’s diary recovered from the battle field on the morning of 20 August 1388.
Since 1752 in the United Kingdom, the Gregorian calendar has been used. Interestingly, the 19 August 1388 in the Julian calendar is the equivalent of 27 August 1388 in the Gregorian calendar.
So, the 634th anniversary of the Battle of Otterburn (assuming that it took place by the light of a harvest moon on the night of 19 August 1388, according to the Julian Calendar is 27 August 2022.