It's up to you. Certainly no code for this concert. If you feel comfortable wearing street clothes to an event where a lot of people will be dressed up-go for it!
As it's the first night, you may find a few people in real formal attire.
In November, when I was in Paris, I attended a performance at the Paris Opera (Palais Garnier), I wore a corduroy sports jacket and a tie. (I chose that because corduroy doesn't wrinkle in a suitcase.) Although the performance was a very dressy affair, I didn't feel out-of-place.