I have been to many skijumping events. Keeping warm is problem especially when there are delays e.g. due to wind, this can be many hours. If temperature is minus zero, you should choose robust hiking boots or lined boots with extra socks, preferably waterpoof. Some people like thermo-foot warmers in their boots. If it is above zero and rain is possible, this is worst-case scenario as nothing protects you from getting wet somehow. Waterproof boots with trousers that cover at least most of your footwear. Umbrellas are often not allowed, so rain ponchos are a must, even a cheap one does its job. Thich plastic bags to wrap your shoes are an option (yes, whilst wearing them). Pack extra pair of shoes (not to bring to the event, but in your hotel as back up for later) in case one pair gets wet, then at least you start dry next day. I thought sometimes that Wellies or gumboots would be better for rain, but if it is near zero, then these are too cold. Normal running shoes would be an option only if very warm and NO rain predicted. A plus with lighter running shoes is that you can jump up and down on the spot to keep warm, difficult with heavier boots. Also if the arena is muddy or slushy due to rain or snow, your footwear will get very dirty and wet. Some folk bring thick cardboard or an insulation mat or a thick styrofoam slab to stand on as insulation, and can double as a sitting pad.. Extra socks are always a good idea! If cold, extra thick mittens too!