Holy Week starts from Palm Sunday, and ends on Holy Saturday.
Christians first celebrated the resurrection of Christ before the observances from Palm Sunday through Holy Saturday were added. This seven-day long observance eventually became the Holy Week that we know today.
The period from Good Friday to Easter Sunday is called the Easter Triduum (three days), where usually ‘The Great Silence’ is observed. This ends with the Easter Vigil.
Easter Sunday starts the Great Fifty Days, marking the time of the risen Lord with his disciples prior to the Ascension and outpouring of the Holy Spirit at Pentecost. Easter — the resurrection — looks forward to what is coming, not backward to what happened. It reminds us to look forward to His Second Coming, as well.