They're going away tomorrow. They'll be away _____ Sunday.

- The correct answer is: Until  

Until
- We use until to mean the end time, up to a specific time or a point in time.
Example: I will work until 5p.m. today.

অন্যদিকে, 

Since
- Since is typically used with perfect tenses.  
- We use since with the Perfect Tenses to express a point of time in the past up to now.
Example: I’ve known him since I was a child.

From
- We use from to show the time or point in time when something starts:
Example: Tickets for the concert are on sale from Monday.

Source: Grammarly.com