The period from April 23rd to April 30th is when new source strings are introduced and individuals can begin localizing new content, providing feedback on the source content and request any changes. The “Unfrozen deadline” refers to the deadline for submitting changes to the source strings prior to the May 1st string freeze.
To clarify for localizers, it is important to start looking at the new strings when they are released on the 23rd of April, so that any necessary changes can be made before they are locked. Then translation and review can be finalized in the week prior to the translation deadline.
Maybe “Unfrozen deadline” > “Source string updates deadline”?