If an individual or a tzibbur thought it was Rosh Chodrsh on the wrong day and said yaleh veyavo, they need to repeat shmonei esrei for shachris and tashlumim for mariv too, correct?
Answer:
Incorrect, bdieved the tefilla is not repeated.
Sources:
See Mishna Brura 108:38 at the end from the Chaye Adam that once the bracha [and certainly the entire shmone esrei] is completed, one does not go back and repeat. In the middle of a bracha one would go back to the beginning of the bracha [and in the middle of the first 3 or last 3 brachos one would go back to the beginning of that group]. However once completed, there is no going back.
