First, don't put glazed or other slippery tiles on an outdoor porch. You will kill yourself when they get wet or icy. You can find some good looking tiles that are specifically designed to be "non-slippery". Always use a thinset mortar that specifically says "indoor/outdoor" on the bag. You must apply the mortar at a temperature between 10 & 21 degrees C (50-70F), and maintain it at that temperature for three days. This is critical. If the mortar gets too hot, it will dry out too fast and the lack of water will stop the bonding process. Water and shade can keep it cool. (If you are in a very dry climate you might even want to cover it with plastic to avoid evaporation, or cover it with canvas and keep the canvas wet for the three days.) If it gets too cold, and especially if it freezes, the bond will be weakened as well. You can protect mortar from near freezing temperatures the same way you protect your tomatoes, cover it with a cloth. If it suddenly gets even colder, use insulation (even straw) to cover it. Like with all masonry products, the ideal for absolute maximum strength is actually to keep the mortar moist and in the proper temperature range for 28 days; however, this is rarely practical. Keeping it moist and warm for three days will give you most of the potential adhesive strength and resistance to freeze/thaw problems. Less than three days and you seriously jeopardize the longevity of your tiling job. Now if you really want the "last forever" tile job, put down an uncoupling membrane first. This will assure that moisture from below, even slight cracking of the concrete, will not have any effect on the tiles. Check out the DITRA membranes made specifically for this application.