As much as possible shoot during a sunny day. Overcast skies and indoors are not really conducive to Holga shooting since the Holga has a fix shutter speed of 1/100 (the length of time that the aperture stays open). But if you like shooting using colorsplash flash and always have a tripod beside you then by all means shoot in ambient or minimal light.
In my experience of using this camera, most of the best pictures I've taken were shot when the sun is high up in the sky. If you want the colors to pop-out and have crazy vignettes on the sides (like the image on the right), then daylight shooting is the way to go.