It is more often used for a kind of party like "宴" as in your sentence or a certain season of the year.

日语有一个老一套的句「宴（えん）もたけなわではございますが、そろそろお開きの時間となりました（宴会虽然高潮, 快要结束了）」 たけなわ是一个古代单词．它意思原来是　”酒发酵得好甜的时候”，用汉字写”酣（たけなわ）”. 用たけなわ不适当描述现代的东西 OK:春たけなわ OK:戦(いくさ）もたけなわ NG:パーティ／コンサートもたけなわ

This sentence means he is going to give a closing speech even though the atmosphere of the party is now at its climax with almost all the guests most excited.

"たけなわ" means that something is at its peak.