The grotto is open from 9 am to 4.30 pm, but only in favorable sea conditions (with the Libeccio or Maestrale wind, it’s impossible to enter the grotto). From November to March, it’s very rare for the grotto to be open, and it’s closed on December 25th and January 1st.
The entrance fee to the Blue Grotto is 18 euros per person. Remember that the Blue Grotto of Capri is a cultural heritage, so the entrance ticket is paid as if you were accessing a museo