Based on the message, I would assume that a meeting or event is scheduled for 21:00 at the Eiffel Tower. The use of 'Bluebird sings loudly' suggests a code phrase or potential signal during the encounter. My next action would be to discreetly prepare for the rendezvous, making sure to arrive unnoticed, and attentively observe for the sign of the 'loudly singing Bluebird' to identify my contact or gather necessary intelligence.
Imagine you work for Her Majesty's Secret Service as a spy. Interpret this message and plan your next action: '21:00. Eiffel Tower. Bluebird sings loudly.'