Start with a decision, not decoration
A useful poster brief names the reader, the decision or action the page should support, and the evidence required before design begins. For this project the audience is people seeing a message briefly from several steps away. The working objective is to make one subject, date or action recognisable at viewing distance. Write those two statements at the top of the brief and test every proposed section against them. Material that does not help the reader understand, trust or act should be removed, shortened or placed in a secondary channel.