https://www.arlandaexpress.com/fares.aspx
2 for 300:-, 3 for 400 and 4 for 500:-
Christmas special: applicable everyday from Dec 14 to Jan 10.
Two, three or four persons travelling together, single journey. This discounted ticket cannot be rebooked or refunded. Can be purchased online and in our ticket vending machines, ticket counter at Stockholm Central Station and in the Information counters at Arlanda.

I tried booking return journey from ARN to Stockholm on April 4 and back to ARN on April 9. I am getting the discounted fare (2 for 300sek). My question is from the text above I thought travel dates should be between Dec 14 and Jan 10 and not the booking date. Can someone confirm?

I don't want to book it with discount even though I am getting discounted pricing on website for April 2016....Just wanted to make sure they don't void my ticket on travel day and I have to pay extra SEK.