Sentence:  In his senior year, he had run across an old '66 Chevy Super Sport headed for the junkyard, bought it for a song, and overhauled it with his dad's help, turning it into the big red muscle car it was back in its <WSD>day</WSD>. 
Meaning of the word marked with <WSD> tags:  A specified time or period; time, considered with reference to the existence or prominence of a person or thing; age; time.