I have 8-day MODIS Land Surface Temperature data from 2009-2013. I have used Model Builder to extract the LST_day and LST_Night Subdatasets (in different model runs). What I am attempting to do next is take 3, 8-day TIFFS (combined, a month of data) and average them so it comes out to be 1 monthly avg. TIFF for each month from 2009 to 2013. I am unsure how I can select specific 8-day data to average in raster calculator using iterator - is there a way to specify or only select certain data? Is there a way to do this in ArcPy?
In your question you say you have already extracted subsets of data (day and night) but your screen shot shows a model where you are extracting a subset so it is not clear what the intended inputs are.
There is a tool in Spatial Analyst called Cell Statistics that would be a simpler approach for computing an average value for a cell over a set of rasters.
You would need to create a raster iterator to collect the rasters into a list. This would be a sub-model within a model that has the cell statistics tool. If you are unfamiliar with sub-models then look at the Help file here.