Intentions & arrangements.

We use the present continuous tense to talk about the future when an action or situation is part of a:  

• plan
• an arrangement
• intention.

Sentences with this use of the present continuous always have words or phrases which refer to future time or the context leaves it very clear:

• Mary’s studying in the library. (now)
  She’s studying at Bethan’s house this evening. (future)
• Stan’s arriving. (now) He’s just called from the airport
  He’s arriving tomorrow morning about 7.30 (future)
• We’re driving to Manchester. (now)
  We’re driving to Glasgow next week. (future)
• We’re doing a physics exam. (now)
  We’re doing a History exam in two days. (future)
• They’re swimming in the pool. (now)
  They’re swimming across the bay tomorrow. (future)