A Day for Love by Mary Balogh

The Substitute Guest
Claire Ward is invited at the last moment to a Valentine’s house party when another guest withdraws. She soon feels very out of place, however, when she realizes the party is intended to be something wild. When each gentleman draws the name of the lady who will be his Valentine for the next three days, the Duke of Langford, the worst rake of them all, finds himself deliberately but quite inexplicably choosing the card of the lady he suspects is prim and virtuous—and very unlike any other he has known.

Golden Rose
The Master of Ceremonies in Bath has devised an intriguing plan for the annual Valentine’s ball at the Assembly Rooms. Each gentleman is to send an anonymous favor to the lady of his choice and she is to wear it or carry it to the ball to discover the identity of her Valentine. But when Roger Bradshaw sends a golden rose to the lady of his choice, his mischievous young cousin, entrusted with delivering it, presents it to another young lady he considers more worthy.

A Waltz Among the Stars
Viscount Brandon, determined to put behind him his grief over the death of his young wife two years before, accepts an invitation to a Valentine’s house party, where he plans to woo the young daughter of the house. However, he finds himself drawn instead to her older sister, who nurses a deep grief of her own while also living apart from her family under the disgrace of a youthful indiscretion.