Code

PropertyInfo imagesInfo = typeof(Calendar).GetProperty("Images",
    BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);

var resourceCache = imagesInfo.GetValue(_calendar, null);
FieldInfo repositoryInfo = resourceCache.GetType().GetFields(
    BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic)[0];
var repository = (Hashtable)repositoryInfo.GetValue(resourceCache);

repository["MoreDown"] = customMoreDown;
repository["MoreUp"] = customMoreUp;
repository["MoreLeft"] = customMoreLeft;
repository["MoreRight"] = customMoreRight;