Butter should be kept in the coldest part of the refrigerator (32º - 38ºF) in its original container or in a covered container. The butter keeper in the door of the refrigerator is NOT the best place to keep the butter as it is generally warmer than the rest of the refrigerator.
Butter (including whipped butter) is "firm" when it is stored under normal refrigeration temperatures (32° - 38°F) and needs to be softened in order to be spread easily. The best method to soften butter is to remove it from the refrigerator and let it warm to room temperature for ½ hour to 1 hour or until it reaches the desired consistency (see Softening Butter). Bring out only the amount of butter that you plan to use for each meal. After use, any remaining butter should be covered or re-wrapped and returned to the refrigerator immediately.
Butter is very susceptible to picking up foreign odors and flavors and should always be stored away from highly aromatic foods. In addition, heat and light can cause undesirable chemical and physical changes in butter. Leaving butter un-refrigerated can result in flavor changes, rancidity, and mold growth.