Members of the public have been invited to join Mrs Maguire's family in a celebration of her life and contribution to education.
The 61-year-old RE and Spanish teacher was stabbed in the neck in her classroom at Corpus Christi Catholic college, Leeds, on 28th April.
A 15-year-old boy has been charged with Mrs Maguire's murder and will go on trial in November.
The memorial service is being held at Leeds Town Hall and an overflow area has been arranged because numbers are expected to be so high.
Rev Msgr John Wilson from the Diocese of Leeds will lead the service, which will feature music, videos and stories from Mrs Maguire's life.
A minute's silence will be observed at 12.30pm.