The phrase "first day" refers to the beginning or initial day of something, like starting a new job, school, or a project. It marks the start of a new experience or journey. Full definition
So much of first day of school lesson plans focus on classroom rules and procedures. (spellingcity.com)
Some red tail style bridal outfits are displayed in our gallery which are best for the first wedding day, as red is usually used on first day of wedding. (latestasianfashions.com)
I'll never forget the night I sat on my living room floor counting the proceeds from my very first day in business. (internationalliving.com)