It's often used to emphasize the length of a period of time or the persistence of a situation.
Here are some examples of how it can be used:
* "He estado esperando por ti hasta el sol de hoy." (I've been waiting for you until now.)
* "Hasta el sol de hoy no he podido encontrar trabajo." (I haven't been able to find a job up to this point.)
Essentially, "hasta el sol de hoy" is a way of saying that something has continued for a long time, emphasizing the current moment as a point of change or resolution.