Without a doubt the worst fishing condition to fish in is a day with high pressure with little or no wind. Fish in these conditions do not move about very much. This means they don't burn off energy, energy they would need to replace by eating. Still, hot sunny days may be nice to fish in but it will make the fish lethargic and docile. By far the best conditions are low pressure fronts.
In the UK this means southwesterly winds which nearly always carry with them wind and rain.
This wind and rain stirs the water creating extra oxygen and stimulating all fish into activity. Fish will normally follow a new wind, especially a new warm south westerly. The part of your lake which is receiving the full force of such a wind is the best place to head for. Many carp will be in these areas as they know the new wind will push food items through the layers to end up very close or even in the margins.
Of coarse there are always exceptions to this rule, for instance, in an over stocked, heavily fished day ticket water the fish may have been forced to change their natural behaviour due to angling pressure. On large lightly fished gravel pits this rule defiantly applies and should always be followed.
In winter when the waters turn very cold carp will lay up together for long periods, sometimes in very large numbers. They still need to feed even in the coldest conditions although only for brief periods.
Air and water temperature are perhaps the most important things to consider. A water temperature's below 5 degrees will hamper your chances of catching. This is not a hard and fast rule however, as some fish will happily continue to feed even when a lake or river may be frozen. The chub for example will feed in the lowest of temperatures and seems unaffected when other fish will be. While the fishes metabolic rate will slow in cold temperatures they still need to feed to survive and this means there is always a chance.
Many believe that fish do not feed in the rain. This is just not true. Rain does affect fish by creating more oxygen which in turn will encourage the fish to become active and so as they burn off energy this will need to replace this by eating more.