First take a look at this question: Bash or GoogleCL: new line in a string parameter
I want to add a variable ${date} into the \"summary\" now:
googl
I'll add yet another option to the list: define a variable as newline, then use that inside double-quotes.
nl=$'\n' ... --summary "Today is ${date}. Poor whiskers takes a tumble.${nl}She's fine, though, don't worry."