Atque omnium primum ad cursus lunae in duodecim menses discripsit annum, additis Ianuario et Februario. Idem nefastos dies fastosque fecit, quia aliquando nihil cum populo agi utile futurum esset.
Livy (adapted by Orberg) again : The second sentence is tricky and is, I believe, pure Livy. Idem means also here or He also..... ?
This concerns the institution of feast days/ days of judicial decision (if there's an English phrase for this I don't know it) and non-feast days / days of judicial decision.
How about (and I'm translating idiomatically): Also he determined days of feasting and judgement and days not for feasting or judgement, because it was not accorded to the people that, at any time, they could decide about their future.
Futurum is a future participle but futurum esset? ...'what could be in the future'?