Sentence:  It was a <WSD>day of days</WSD> for California railbirds. Not only was it the day of the $100,000 Santa Anita Handicap, world's richest horse race, but this was the now-or-never race for doughty old Seabiscuit, darling of U. S. racing fans, Cinderella of the turf. 
Meaning of the word marked with <WSD> tags:  A particularly noteworthy day; the day on which a milestone or especially memorable event occurs.