House of Justice would legislate on details related to the obligatory prayers - Shoghi Effendi

First, with regard to your questions concerning the obligatory prayers, the "Aqdas" does not give detailed instructions about them. The Universal House of Justice, however, will have to define the exact time for their recital, and lay down, if required, other detailed instructions concerning their use. One of the three obligatory prayers should be recited. This is an obligation. But individual believers are absolutely free to choose any one of them. There is no particular time of day fixed for the reading of the long prayer.

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