Please note; it is not “The Big Day”.
Capitalized “Big Day” connotes a day of exceptional importance. Such “Day”s are Wedding Days, Birth of Children Days, Graduation Days. Lower case “big day”s are some what special, out of the ordinary days; such as...oh, let's say a 42nd birthday. While special, it is not quite as momentous as a Milestone Birthday. Other examples of lower case “big day”s are buying a new car, bagging a first deer, your child losing their first tooth, and landing your first real job.