Due to the specialist's speech（lecture） on 20th pf April in the school, the English speaking competition will be delayed（rescheduled） until 15th of May.或者妳可以使用倒裝句（The English speaking competition originally hold on the 20th of April will be delayed（rescheduled until the 20th of May due to the specialist's lecture-visit to our school.）
We （sincerely） hope that all the member of staffs in the school will inform all the participants "as soon as possible"(盡快的). Please reply a confirmation letter.