create reusable modules?

Hi all, is there a way to create a piece of an email that can be added in automatically to my main email? I send emails every day. I have certain content that needs to be added every single week, but only once per week and monthly reusable content as well. I have 20 or so such "modules". Maybe some kind of placeholder that I can link to modules/blocks and say "block 1 goes here on Mondays" "block 2 on tuesdays", "block 15 third wednesday of every month", etc? Thanks!