I wanted to clarify whether comma usage was mandated, optional, or not allowed in cases where the phrase at the beginning could be moved later into the sentence. I realize this is not the best description so let me give two examples:
The next day [?] I went to the beach. (I went to the beach the next day.) Together [?] we drank juice. (We drank juice together.)
In both cases, the section on the left can be moved into the sentence where a comma would definitely not be needed (and sound awkward).
My feeling is that for the sentences on the left, in both cases a comma is optional, but not necessary. Is this correct? If the first sentence was "The very next day", would it change things? I've heard for some phrases if the length is above three words then a comma is recommended.
EDIT: While someone has indicated this a possible duplicate, I feel the referenced article does not clearly answer my questions here. You can see from the comments in this post the answer is not trivial.