Firstly, you can never have too many clothes for Rowany Festival. I know people who have been going to Rowany Festival for a decade and still dont have enough clothes.
The absolute minimum needed is two sets of garb for cold and/or wet weather, and two sets for hot weather. Some people take two changes of clothing for every day. Most people have problems getting enough cold weather gear, as for reasons of finance, much SCA clothing is made from cotton. Lincraft often sell cheap wools and wool blends in March (the previous winters stock), so look around and try to get some wool for a cloak or to make some warm surcoats. Remember that if it is raining, you will probably get your clothes wet, so you need to have more than one set of clothes for cold, wet weather. If you dont have enough warm clothes, wear thermal underwear, a T-shirt or tracksuit pants underneath a tunic. To keep warm, wear several layers of garb- wear two tunics or a surcoat over a tunic.
One cloak is an absolute necessary. Two cloaks are a bonus. After several hours of continuous rain, most cloaks will have absorbed so much water that they weigh a tonne. Having a spare cloak gives you something else to wear while you hang up the first cloak in a large tent and hope that it will dry. A cloak also makes a good extra blankie for your bed, as well as giving you something to wear when you have to get up in the middle of the night to find the privies. Make at least one of your cloaks from wool if possible- wool is warm, somewhat water resistant, easy to sew with and it looks great. A cloak can be as simple as a rectangle or semi-circle of wool.
If possible, pack a clean chemise or undertunic for every set of garb that you take. There is no point in having six new dresses if you have to wear the same dirty chemise under all of them. Wash and dry your underwear while at Rowany if necessary and the weather is warm enough.
Hats and headwear.
Firstly, a hat will keep you warm at night (especially hoods), while a sunhat or veil will help to prevent sunstroke on hot days. Secondly, hats or veils can hide a bad hair day- important if you havent washed your hair for three days! And lastly, hats get attention! Make a new dress and a few people might notice. Make a flashy new hat and wear it with an old dress, and suddenly everyone will comment on your new outfit!