Just sharing my thoughts..
Previous clues gave (B)ourges, (A)Ronceavux, (N)apoleon Bonaparte (C)herbourg
With this the first letters it will turn into BANC...
And what is wierd is that there is a Bank which is exactly beside the OPERA HOUSE in Bordeux, It was named Banque Courtois...
Could this be what is meant by SIDEreal Day?
Look at the Enigma #580,
The BAGFEDC clue which shows a letter "bc" hidden behind the letter G. Also notice the whole Image, statues with a golden light.. For me this is the Grand Theatre de bordeux... Sidereal day could mean