The 120 days is for Annual Passholders. The actual wording on the WDW website on the
DAS FAQs is:
"DAS is valid for the length of the ticket or up to 120 days, whichever is shorter. Once the service has elapsed, Guests need to re-register for the program."
On the general DAS information, it just says:
"DAS is valid for up to 120 days"
As jennab mentioned, if you have APs and were approved under the new rules (after May 19 for WDW), DAS will expire in 120 days. If you don't have APs, it expires at the end of your tickets.
I don't know for sure how DAS registration will work this year for the parties. Last year, I saw some reports that people needed to re-register for the party because it was a different ticket. Some had ONLY party tickets, so needed to register since they were not previously registered.
Guests can arrive for the party starting at 4pm.
According to reports from last year, DAS Return Times were available in My Disney Experience app until the park closed to other guests at 6pm. It was not available during the 1 hour time between park closing at 6pm and the party starting at 7, but guests could make DAS Return Times by going to the attraction they wanted. Once the party started, guests were again able to make DAS Return Times in the app.