Here’s what an actual moral theology manual hasn’t to say about the matter:
- Fulfilment of the obligation.
In itself this precept obliges us to pray frequently during life, but it is impossible to determine accurately how often we are bound to pray in virtue of divine law. It is the common teaching of theologians that the precept in itself binds us to pray ; 1. at the outset of man’s moral life, when man who has attained to the use of reason must first turn himself towards God; 2. in danger of death ; 3. frequently during life.
But for incidental reasons the precept obliges us to pray : I. each time that other precepts cannot be fulfilled without prayer, such as the precept of hearing Mass on Sundays ; 2. in moments of temptation to grave sin which cannot be resisted without prayer ; 3. in serious disasters, especially of a public nature. (D. Prummer, 1955, Handbook of Moral Theology , my emphasis)
It seems that according to this source, there is no strict moral obligation to pray even calendar day.
That being said, it is certainly recommended to pray every day, and indeed we are called to “pray without ceasing.”