It isn't really a special promotion, it has been available for a while now (as I've said used it last year).
From Finnair site:
PURCHASE LOUNGE ACCESS IN ADVANCE
As a Finnair customer you can buy access to the Schengen-area and non-Schengen area Finnair lounges for a reduced price when your flight departs during off-peak hours.
• To get the discount, your flight must leave between 6 a.m. and 3 p.m. or 6.45 p.m. and 12 midnight (Finnair Lounge in non-Schengen-area), or between 10 a.m. and 3 p.m. or 7 p.m. and 12 midnight (Finnair Lounge in Schengen-area).
• You can buy lounge access via Manage Booking or by contacting our customer service.
• Both lounges serve hot breakfast daily before 10 a.m. and warm meals from 3 p.m. to 5.30 p.m.; in the non-Schengen lounge also from 9 p.m. to 12 midnight.