I also thought that was odd and I also got that wrong - because anniversaries are also related to people in English. However maybe it is more common that anniversaries are usually between two people or entities, such as 'Their anniversary is in July' or it is 'her anniversary in July with the company'. It could be that this specific phrase in Portugese only ever relates to Birthdays and that is something we have to memorise.
It is not Simple Present Tense:
"Will be" is Simple Future Tense which we have not learned yet at the levels that this exercise is placed in the DL lessons.