Basic principles to remember when thinking planning and forecasting. If these are not all met its likely your forecast will not be fully accurate and indeed its difficult for accountability as its unrealistic to assume accuracy when you do not have access to all of these.
The easy one, make sure you have all the appropriate historic information. Always ensure you understand not only the volumetrics but also the equivalent FTE output for each period as its never apples to apples.
This is really important and it can be a massive issue in some centres though equally it may not be necessary to take into account. Really this is about ensuring targets are being met by Operations, e.g. Occupancy, Wrap, Conformance etc. If they are not, any root cause analysis is likely to show them as the main cause for Forecast inaccuracy.
Very often lacking, knowing in advance where appropriate business changes that may impact you, could be a CEO talking about a particular service, a company update etc. These are usually quite hard to define and are easiest to obtain from the C-Suite or Senior Directors.
An obvious one, make sure you are working in collaboration with Marketing (or equivalent if Public Sector) to understand changes, upcoming promotions etc.