What is the earliest event a cookie written in HEAD tag by javascript can be accessed in .net page life cycle?

I have javascript writing a cookie just after the HEAD tag. I can retrieve the cookie with vb.net on the postpack of the page, but not on the Page Load event of that initial rendering. Is there a way I can write the cookie if js and access the value in .net in that same initial rendering?

I think I need a trigger event fired after page load (or a delay?). I have also tried writing the value to a text field and reading it from there, but the value is not there  yet when the code checks, but it is there when the page is finally loaded, so it's pretty clear I'm asking too early in the cycle.

IMPORTANT. This value is used to load custom images on the screen, so it needs to be as early as possible.