Total UK sales were barely changed, up 0.1% compared with the same month last year, while like-for-like sales, which exclude new store space, fell 1.0%. The bank holiday was on 31 August, but both the BRC and the Office for National Statistics judge that the month officially ended on 29 August.
It means Septembers figures will be boosted by back-to-school purchases.
The bank holiday applied in England, Wales and Northern Ireland.
Retailers report their sales on a weekly basis from Sunday to Saturday, which means that monthly figures do not necessarily cover the whole of a calendar month. Instead, a quarter will be made up of two four-week periods and a five week period.
This is only particularly important when key shopping days such as bank holidays officially fall in different months from year to year, which makes comparisons difficult. Clothing and footwear sales were both hit through missing the key days of back-to-school purchases, but food sales were up 0.3% in the three months to August.
At this time of the year parents are busily shopping for back-to-school essentials like clothes, footwear and stationery and those sales will peak later this year, said BRC director general Helen Dickinson.
Large-ticket item categories like furniture and household appliances also experienced a decline in sales, again likely affected by the bank holiday distortion.