expression
to one's delight; happily
Fixed phrase used adverbially to express that something pleasant happened. Often placed at the beginning of a sentence.
嬉しいことに、明日は休みだ。
To my delight, tomorrow is a day off.
嬉しいことに、彼から手紙が来た。
Happily, I received a letter from him.
From 嬉しい (happy) + こと (thing) + に (adverbial particle), literally 'to a happy thing'.