'splendid days' is correct and usable in written English.
It is usually used to express fond memories of something or to describe something that is very good. For example, "I remember our summer vacation - it was filled with splendid days at the beach".
Tiger Woods has enjoyed many splendid days at the Bay Hill Invitational, but Friday was not one of them.
Two splendid days they were, in which friends came and went, staying for lunch, dinner, or simply a glass of something pleasing as Arlott talked about the things that intelligent people like to talk about.
These normally splendid days on the range -- the very days when gun dealers and firearms aficionados are preparing for the annual gun show over in Frederick this weekend -- are being sullied, though, by the unnerving sniper situation.
The electronic urban festival featured major artists in the realm of electronic, hip-hop, trap, rap, house and a whole lot of dancing in the rain across two splendid days of partying, good vibes and muddy escapades.
— It's a splendid day for a stroll past a haunted house: dark, damp, forbidding.
BY ANY standards June 9th was a splendid day for Turkey's 12m-odd Kurds.
Mrs. North parked the car and came in and told Mr. North what a splendid day she had.
I am at home and from my window I can see the San Francisco Bay Area on a splendid day, two bridges, redwoods, oak and bay trees, seagulls and sailboats.
This question arose at Words and Wickets, a splendid day at Wormsley that combined cricket and literature into a potent draft.
In the gutter, rivers raced along, divided into streams, formed rapids and waterfalls, diverted into lakes and ponds... Thin oil slicks slid along the gutter, silver blue, folding into paisley patterns, cardiograms, shot silk, marbled bookbindings, Art Nouveau... We saw the reflection of blue sky on the surface of the final lake, and, looking up, observed that it would be a splendid day.
It was quite splendid, and revealing of how bankers felt about themselves in the glorious days of the mid 20th Century.
