I'm really confused about adding a comma before an introductory phrase. Here's my original sentence:
"Yesterday I found Susan's missing cat."
I wanted to add the introductory phrase, "While I was getting the mail," after "yesterday," but I didn't know if the sentence should be written like this,
"Yesterday while I was getting the mail, I found Susan's missing cat."
Or,
"Yesterday, while I was getting the mail, I found Susan's missing cat."
Is a comma required after "yesterday" when an introductory phrase, such as "While I was getting the mail," or is it preference, just like how it's preference to have a comma after words such as "today," "yesterday," "now"?