Current questions and answers regarding the map editor.

local SET_LIGHT_GROUND = { 'Mo', 'Tu', 'We', 'Th', 'Fr', 'Sa', 'Su' }
if mod(GetDate(DAY), 3) == 0 then
local lightIndex = mod(round(GetDate(DAY)/3),7)+1
SetAmbientLight( GROUND, SET_LIGHT_GROUND[lightIndex], nil, 0 )
end

doesn't work either. It says, value was nil when getting global with name "round"
attempt to call a nil value

The map uses its own files ;)
'Mo', 'Tu', 'We', 'Th', 'Fr', 'Sa', 'Su'
Copy these files to the root of the map and check at least "Tuesday" ;) (and there will be no xdb#xpointer(/AmbientLight) there). You can also easily take the lighting you want from the resources and put it in the map.
"round" is rounding, it probably won't be needed, so don't use it (or better, use 3.1 ;) )