Postings in SAP are controlled by the ‘opening’ or ‘closing’ of posting periods. Normally, the current posting period is open for document posting and all other periods are closed. At the end of the period (month), this posting period is closed and the new one is opened for postings. This way it provides better control.
It is, however, possible to keep all the periods or select periods open. Transaction Code for the same is OB52.