I've got dozens of seedlings that I've been lugging in and out of the garage for the past few months. Temps at night are starting to stay at around 42F to 50F as night time lows and the high 70's in the day. Would this be too cold to leave them out over night yet? Also, I'm putting them in direct sunlight during the day, should I leave them in shade?

Another night in the garage I guess. I just don't want to lose any.

If your trees have already acclimated to the sunshine, then you should leave them in the sun. Sunlight is the ONLY source of energy for a citrus tree. As the irradiance (light intensity) increases, photosynthetic CO2 uptake increases, the trees food supply increases, therefore the growth increases. Shaded leaves have a 30 to 50 percent lower respiration rate than sun leaves, irrespective of temperature and leaf age. If the night temperature in your area stays in the 40 and 50s your seedlings should be safe. However, if the night temperature inside the garage is warmer, say in the 60+ range, then generally between the outside day and the inside night temperatures, your seedling would grow faster. Lastly, the outside wind, up to a point, will give your seedling straight, strong, trunks, therefore avoiding the need for a stake to hold a weak spindly trunk upright. - Millet (682-)