The women went to the tomb very early on Sunday morning. This was the first chance they had to spice Jesus' body to preserve it. Since Jesus died on Friday and Saturday was the Sabbath and so no work could be done, they had to wait until Sunday to get at the body. Jesus needed to be in the tomb for three days before rising for anyone to believe that he had actually died. The Jewish people at that time believed that it took three days for the soul to leave the body. If Jesus rose on the Saturday they wouldn't believe he was ever fully dead.
The women needed a reminder from the angel about Jesus' claims. We see again the need for the Holy Spirit as Jesus had told all of them multiple times that he had to die and then would rise again three days later. Since they did not have the Holy Spirit yet they couldn't understand what he was saying. The angels remind them that he said he would rise again and now he has risen and the women then remember and rush off to tell the disciples the good news.
The interesting thing here is that the angels, and in the other Gospels Jesus, reveal that Jesus has risen to the women and tell them to go tell the others. During this time women were second class citizens. A women's testimony didn't even hold up in a court of law. But Jesus uses the women as the first representatives of the Gospel. The fact that the Gospel starts so small, 12 disciples and first told through women, and exploded throughout the world really shows the truth of Jesus as the Son of God and what he did on the cross.