The Scarlet Sails festival takes place in Saint Petersburg, Russia during white nights at the end of June. School graduates come to celebrate the end of their senior year. Their families and friends come to congratulate them on the beginning of their adult life. It attracts thousands of people to watch fireworks, light, orchestra, water show and, of course, a beautiful ship under scarlet sails. The romantic spirit, celebrating love, dreams and hope is the basis of the festival.

Решение:

• to celebrate — подходит к фразе "School graduates come to celebrate the end of their senior year." (выпускники школ приходят, чтобы отпраздновать окончание старших классов).
• to congratulate — подходит к фразе "Their families and friends come to congratulate them on the beginning of their adult life." (их семьи и друзья приходят, чтобы поздравить их с началом взрослой жизни).
• to attract — подходит к фразе "It attracts thousands of people to watch fireworks, light, orchestra, water show..." (это привлекает тысячи людей посмотреть фейерверки, свет, оркестр, водное шоу...).

Ответ: to celebrate, to congratulate, to attract.