Who was the first to know of Christ's Resurrection?

Here is what Saint Gregory Palamas says:

There is something which the evangelists tell us in a veiled way, but which I shall reveal to your charity. As was right and just, the Mother of God was the first person to receive from the Lord the Good News of the Resurrection, and she saw Him risen and had the joy of His divine words before anyone else. She not only beheld Him with her eyes and heard Him with her ears, but was the first and only person to touch with her hands His most pure feet. If the evangelists do not say all this openly, it is because they do not want to put forward His Mother as a witness, lest they give unbelievers grounds for suspicion. As we are now, however, by the grace of the Resurrection, addressing believers, and the subject of today's feast obliges us to clarify everything that concerns the myrrhbearers, this too shall be revealed, with leave from Him who said, "Nothing is secret, that shall not be made manifest" (Luke 8:17).

Who were the Myrrbearers?
These are the women who followed the Lord along with His Mother and stayed with her during the time of the Passion of our Lord.  They were the ones who wrapped Christ's body in linen cloths with glue-like spices, and put it in the sepulcher.  


When there was an earthquake and the tomb was opened, the Theotokos was there. She did not fear as others ran and the angel came saying "Fear not ye: do ye seek Jesus which was crucified? He is risen. Come, see the place where the Lord Lay" (Matt 28:5-6). They then departed with "great joy" (Matt 28:8). Later others came to the tomb, some rejoining the Theotokos and went to the place e they aw and heard Him speak to them.


For complete description of the most significant event see the Eighteenth Homily by Saint Gregory Palamas, "On the Sunday of the Myrrhbearers," in Saint Gregory Palamas: The Homilies pp 144 - 151.