Sentence:  Who could have believed, that saw her on the night of the <WSD>Mardi Gras</WSD> amid the revels at the Palace, seemingly joyous and happy as youth and innocence could make her, far excelling in loveliness all the fair dames gathered round her, that envy and hatred, and hot desire of revenge, were all hid beneath that seemingly guileless smile and treacherous "abandon"? 
Meaning of the word marked with <WSD> tags:  The last day of a carnival, traditionally the celebration immediately before the start of Lent when joy would be out of place for Christians.