This is the 1290th most frequent Japanese word.
楽しく
"楽しく" means "joyfully" or "enjoyably."
In this sentence, '楽しく' is an adverb describing how the action '過ごせました' (spent) occurred, implying that the time was spent in an enjoyable manner.
昨日は本当に楽しく過ごせました。
Yesterday, I really had a joyful time.
Here, '楽しく' is used as an adverb modifying the verb '歌う' (sing), indicating that the singing is done in a cheerful or enjoyable way.
楽しく歌を歌うことが好きです。
I like singing songs joyfully.
In this context, '楽しく' describes the manner of the communication ('話しました' - talked), emphasizing that the conversation was enjoyable.