So far, this appears to be a one-time offer that expires at the end of the year. The company has far and away the largest streaming subscriber base with over 60 million subscribers as of July, 2017, double what Apple Music has (Spotify hasn't released numbers since then). In June of this year, Apple quietly unveiled a permanent $99 per year subscription for existing users. Amazon Music Unlimited, meanwhile, is $7.99 for Prime members, which works out to about $96 per year.
Spotify recently made changes to benefit artists, limiting song access in the ad-supported version, and buying Mediachain, a company that helps it figure out who owns royalty rights. Despite the subscriber numbers and $2.2 billion in revenue last quarter, Spotify still ran an operating loss of around $120-240 million last quarter.
Given that, it makes sense for Spotify to launch a yearly streaming deal to get a shot of revenue over the holidays. If you're an existing Premium user or having been thinking of signing up, you can grab the deal here until the end of the year.