Day, who has built an audience via social networking, has achieved the feat without a record label and minimal radio and TV support.
All of his sales have come via digital download sites, arranged by 'Nermie Army HQ'. His UK sales tally - revealed by Official Charts Company data this evening - is big enough to fend off Coldplay's Paradise, which takes the No.5 slot on the Christmas Chart behind Day with just over 50,000 weekly sales.
Day's video for Forever Yours, hosted on friend and fellow YouTuber Charlie McDonnell's hugely popular channel, has pulled in over 2.7 million views. Day's own YouTube channel - which includes videos featuring his musings on The Bible and teen vampire series Twilight, as well as his music - boasts over 480,000 subscribers.
As reported earlier, The Military Wives take the No.1 spot in the Official Singles Chart, with over half-a-million sales. Winning X-Factor group Little Mix's Cannonball holds to claim second spot with over 116,000 sales, followed by the Chris Moyles-backed release Dominick The Donkey at No.3, with 92,731 unit sales.
Despite a strong showing in the earlier Midweek charts, Nirvana's Smells Like Teen Spirit misses out on a Christmas Top Ten slot. The classic grunge song finishes up at No.11 with 30,435 unit sales in the week.
Official Charts Company MD Martin Talbot said: "The week before Christmas is always an incredibly competitive sales week, so for Alex to make such a big impact in the Top 10 without the support of a traditional record label - big or small - is truly impressive.
"Forever Yours is certainly one of the most successful self-released tracks we have ever seen, the Official Charts are based on sales, and sales alone, so Alex's achievements this week are a genuine reflection of the passion of his army of fans, and of course, the power of social media. His success is why the Official Singles Chart truly is 'the people's chart'."
As expected, Michael Buble's Christmas LP tops the Official Christmas Album Chart tonight, after shifting 317,114 units in a week - followed by Amy Winehouse's Lioness: Hidden Treasures, which sold 216,868 in the same period.