Sri Radhastami. Srimati Radharani's appearance at noon.

Selected references to this day, on the Math's websites.

Audio. Sri Radhastami, Vrindavan, 1999. Srila Bhakti Sundar Govinda Dev-Goswami Maharaj explains that Radhastami is when Lord Krishna the Supreme Personality of Godhead's divine potency Radharani appears in a form for playing with the Lord. Srila Swarup Damodar Goswami said, "For the divine play of the Sweet Absolute, He wants to divide Himself as Power and Powerful, and that is Radha Krishna." When it was manifested we cannot say because it is eternal, it cannot be counted by our time and space theory, it is achintya, inconceivable. Radharani is the divine potency of Lord Krishna and She is giving all the happiness, love and all the affection to everyone for the service of Lord Krishna.
Improved quality audio of Srila Gurudev speaking by phone from Vrindavan to Madhusudan Prabhu in Hawaii on Sri Radhastami, 18 September 1999:
7+ minutes, 1.8 MB mp3 audio

Our Lord's Beloved Power. "Today is Sri Radhastami, the Appearance Day of Srimati Radharani. In the Scriptures, especially Sri Chaitanya Charitamrita there is much glorification of Srimati Radharani ... Where we see the supreme manifestation of madhura-rasa, there we see the manifestation of Srimati Radharani."
Chapter 12 of Affectionate Guidance by Srila Bhakti Sundar Govinda Dev-Goswami Maharaj: 150 KB pdf document. 

Radharani's Paramour Love. Srila Bhakti Sundar Govinda Dev-Goswami Maharaj describes the glory of madhura-rasa in Vrindavan. Posted on the Math's gaudiyadarshan.com website.

Audio: Sri Radhastami. Today's beautiful, special morning discourse by Srila Bhakti Sundar Govinda Dev-Goswami Maharaj at our Dum Dum Park Temple, Kolkata. 1 September 2006:
Download, or listen online (46+ minutes, 5.4 MB mp3 audio)
or:
higher quality (10.7 MB mp3 audio)

Audio: Sri Radhastami Discourse 2004 by Srila Bhakti Sundar Govinda Dev-Goswami Maharaj.
"Everything you want to know, you can understand through your Gurudev. And Gurudev's position is always supreme. In the Scriptures it is said, (sloka) 'If Krishna will be angry, Gurudev can save that person, Radharani can save that person. But if Radharani will be angry, Krishna cannot save him.' (Sloka). Then our protection, our environment - everything - through Radharani's mercy if it will come to us we will be so grateful, so fortunate and so good practitioners. We are practising service to Radha Krishna, and that is called Krishnanushilana: practising the procedure of satisfying Radharani and Sri Krishna."
21 September 2004, Soquel, USA:
35+ minutes, 8 MB mp3 audio

Audio: Sri Radhastami discourses 2004 by Spd Goswami Maharaj and Spd Janardan Maharaj at the request of Srila Govinda Maharaj.
Then Srila Govinda Maharaj talks briefly then sings Sri Radhika-stava and Sri Radhastaka. 21 September 2004, Soquel, USA:
40+ minutes, 10 MB mp3 audio