I understand this as, on the same day, there's going to be an announcement of abdication and <some other ceremony and 臨まれた>
I'm not sure whether you're parsing the sentence correctly...
同日午後に「退位礼正殿の儀」の実施を告げる is a relative clause that modifies 「退位礼当日賢所大前の儀」.
臨まれた here is the honorific form of 臨んだ. (れる is the honorific auxiliary/尊敬の助動詞.)
天皇陛下は30日午前、[皇祖神の[天照大神]{あまてらすおおみかみ}をまつる]皇居・宮中三殿の[賢所]{かしこどころ}で、[同日午後に「退位礼正殿の儀」の実施を告げる]「退位礼当日賢所大前の儀」に臨まれた。
In the morning of the 30th, at 賢所 in 宮中三殿, the Imperial Palace, [where the imperial ancestor 天照大神 is enshrined,] the Emperor attended 退位礼当日賢所大前の儀, [where he announced the 実施(holding?) of 退位礼正殿の儀 in the afternoon of the same day.]