Atmost 2 periods ( fiscal periods ) can be open at any given time to post material accounts. Typically at the end of the month ( or whatever the fiscal period is ) , the new period is opened and this automatically closes the last period.
The transaction path to close posting period is [MMPV] and the menu path for the same is [Logistics -> Material Management -> Other -> Close Period ]
Enter the company code ( This could be done in one-shot for a series of company codes ) the new fiscal period to open ( This is normally the next fiscal period ) or you can specify the current date or the date based on which SAP needs to choose the fiscal period to open/close.
Some materials have negative postings enabled. If that is the case and if you are OK to have negative quantities/values allowed in the previous fiscal period, select the “Allow negative quantities in previous period “ check mark .
To find out which periods are currently open go to transaction code [ OMSY ] and select the company code to find out the current fiscal year/period, the previous fiscal period/year etc. Also, the flag highlighted allows posting to the previous period.