Impossible to say with any accuracy because Isthmian League attendance records weren’t kept systematically until the 88/89 season. Lots of info and spreadsheets galore here
scousedom:
The Official Isthmian League Historical Website - Your one stop resource
On the balance of probabilities 3/9/1938 vs St Albans (att: 6000) and 7/9/1938 vs Clapton (att: info unavailable) look likely candidates - albeit with a further 24 hours pause compared with 26-29/12/2018.
Incidentally, John Lawrence’s programme notes from yesterday (29/12) reported the sad news that Mike Wilson, ‘the long-serving and much-respected statistician of the Bostik (Isthmian) League’ had died earlier in December. He had been recording attendance figures for over thirty years.