My everlasting memory (apart from the music) of the ABBA concert I attended - Friday 9 November 1979 - is the scent of hay in the foyer as you walked in. The venue had been used for horse trials before ABBA took up residency. Luckily, it was not so noticeable to me in the auditorium.
Also, the Epic Records stand in the foyer, where they were selling all the ABBA albums released on that label up to that point. I bought my copy of the newly-released 'GHV2' there. It was the only time I had seen large piles of the 'Waterloo', 'ABBA', 'Greatest Hits' etc. albums all stacked up ready for sale...and there was a sign to say that every album was £4.29 (a £1 discount off their current RRP).