We were present at the dedication ceremony of the building.
竣工式（しゅんこうしき）is a synonym of 落成式. I believe you hear 竣工式 more often. The difference is very subtle and there is no clear definition.
But 竣工 is frequently used from a developer’s perspective. 竣工式 is often held to celebrate the completion of construction among carpenters, suppliers and investors. 工 means 工事 = construction.
On the other hand, 落成 is often from an owner’s perspective. Neighbors and/or prospective users of the building are sometimes invited to 落成式 to celebrate the completion of construction.