arcpy.mapping.ListLayers(mxd,"FILTER",df)[0].

import arcpy
raster = arcpy.GetParameterAsText(0)
outPDF = arcpy.GetParameterAsText(1)
hill_bool = arcpy.GetParameterAsText(2)

mxd = arcpy.mapping.MapDocument("CURRENT")
df = arcpy.mapping.ListDataFrames(mxd, "Layers")[0]
hillshade = arcpy.mapping.ListLayers(mxd, "hillshade", df)[0]
hillshade.visible = hill_bool

tmplist = ["Januar","Februar","Maerz","April","April",
"Mai","Juni","Juli","August","September","Oktober","November","Dezember"]
monlist = arcpy.mapping.ListLayers(mxd, tmplist ,df) #Select only the months
addLayer = arcpy.mapping.Layer(r"C:PythonLegende_2.lyr")
arcpy.mapping.MoveLayer(df, monlist[:-1],hillshade, "AFTER")

for element in monlist:
    element.visible = "TRUE"
    arcpy.ApplySymbologyFromLayer_management("","addLayer")
    if hillshade.visible = "TRUE":     #<<< ???
        element.transparency = 35
arcpy.RefreshActiveView()
...

#...align Legend and other objects -> ExportToPDF`
arcpy.mapping.ExportToPDF(mxd, outPDF)