September 25

As regards your question concerning the Nineteen Day Feasts: this is really a matter of secondary importance, and should be decided by the Assembly; meetings which have been publicly advertised for a certain date cannot, obviously, be cancelled. As to non-Bahá'ís attending: this should by all means be avoided, but if non-believers come to a Nineteen Day Feast, they should not be put out, as this might hurt their feelings. 

- Shoghi Effendi (From a letter written on behalf of Shoghi Effendi, 21 September 1946 to two believers; The Compilation of Compilations, vol. I, The Nineteen Day Feast)