Day has not played since round three after suffering a stress fracture in his foot.
With just five rounds until finals, star midfielder Day's return is a timely boost for top-four hopefuls Hawthorn.
The 23-year-old will be out to make up for lost time, having been kept to 16 games last year due to injury.
He had averaged 23 disposals and 5.3 clearances this year before his foot problem arose.
Key forward Mabior Chol faces a test to return, while question marks remain over Josh Weddle (back) and Finn Maginness (kidney).
As the club awaits scans on Weddle, Maginness appears unlikely to feature again this season after requiring surgery for a kidney laceration following the Hawks' win over Port Adelaide at UTAS Stadium.
"Finn's still down in Tasmania with family and seeing specialists and getting the best advice," Hawthorn high performance boss Peter Burge said.
"We'll hopefully get him back to Melbourne towards the end of this week."
McKay is set to be able to turn out to farewell retiring club champion Sam Docherty after recovering from minor surgery for a nagging knee issue.
The 27-year-old, who hasn't played since round 11, is available alongside Lewis Young (ankle).
Elsewhere, Gold Coast star Daniel Rioli will undergo surgery to his left fibula ahead of the QClash against the second-placed Brisbane Lions.
The three-time premiership Tiger, who joined the Suns this season, could miss up to six weeks.
Rioli, who injured the medial collateral ligament in his left knee during pre-season, hurt his leg in the second quarter of the Suns' 61-point loss to Adelaide.
Gold Coast, still chasing a maiden finals appearance, have slid to eighth on the ladder after being held goalless in the opening half by the Crows.
Rioli joins midfielder Touk Miller (hamstring) on the sidelines.
Miller, whose return timeline has been adjusted to "short-term", is expected to remain out of action for two weeks.
Gold Coast could at least welcome back Sam Collins (calf) and Lachie Weller (cork), while Brisbane veteran Dayne Zorko (foot soreness) is available to play.
Melbourne's Tom Sparrow is also on target to make his return against St Kilda, after recovering from a concussion he suffered in a contest with suspended North Melbourne ruckman Tristan Xerri.
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