In 1865 the Freemasons' magazine gave the following answer:

The Medieval Lodge was a frame structure placed on the southern side of the building to be erected.  It had the advantage of the light and warmth of a southern aspect.

It had three main windows - in the East, the West and the South.⁽¹⁾  Originally these windows where termed the 'Three Great Lights".  They were represented on early tracing boards, and were alluded to in old rituals of 1725 and 1730. Later they were termed 'fixed lights',  their uses being 'to light man to, at, and from work'.  It is expressly stated that these lights 'are three windows, supposed to be in every room where a Lodge is held.'

At the three windows are seated the Master and Wardens.  The Fellowcrafts had their appropriate stations, and the Apprentices where placed in the north, as they required less light than the more skilled and advanced Fellowcrafts.

When sufficient Light came through the window in the east, the Master called the craftsmen to labour.  When the sun reached it's meridian in the south, the Junior warden called the craftsmen from labour to refreshment, and on again to labour. When the sun was setting in the west and there was not sufficient light to work, the Senior Warden gave instructions to cease labour.

When brethren met for social purposes or business at night, candlelight became necessary; the Officers retained their usual positions, and before each was placed a candle.

(1) The Lodge building would most likely have been a 'lean to' or temporary structure, so if  built on the south facing wall of the building under construction, a window in the north would have been either impractical or impossible.