Earth Day is on the 22nd of April. WebEnglish.se has lesson plans and materials in two levels: Environment 5-7 and Environment 8-9.
The Literary World is celebrating the Bard on April 23rd, which presumably was Shakespeare’s birthday as well as the day he died in 1616. WebEnglish.se has intermediate-level materials divided into two categories: Shakespeare’s life and  Shakespeare’s Work.

For young learners (yr 1-6), there is a theme page of Spring to fill the April lessons with.