The Latin tag in this sonnet means ‘your light is my way‘. Belloc was Catholic, after all, in an era when the mass was in Latin, and most educated people could read it.

Three

Rise up and do begin the day’s adorning ;
The Summer dark is but the dawn of day.
The last of sunset fades into the morning ;
The morning calls you from the dark away.
The holy mist, the white mist of the morning
Was wreathing upward on my lonely way.
The way was waiting for your own adorning
That should complete the broad adorned day.

Rise up and do begin the day’s adorning ;

The little eastern clouds are dapple grey :

There will be wind among the leaves to-day ;

It is the very promise of the morning.

Lux Tua Via Mea : your light’s my way-
Then do rise up and make it perfect day.