The Australian World No.1 carded a one-under-par round of 70 Friday at the at the Deutsche Bank Championship in Boston but left immediately after being told of his family's road accident which happened nearby.
"After the completion of my round today I was informed my wife Ellie and children Dash and Lucy and friend Katie were involved in a traffic incident," Day said in a statement posted on his Twitter account.
"Their car was hit by a bus and Ellie was taken to the hospital for precautionary measures. Thankfully everyone is now resting comfortably and Ellie has been released.
"They will all be monitored in the coming days. We want to thank the emergency services and hospital personnel for their assistance."
According to the official PGA Tour website, Day should return Saturday to carry on in what is the second of four tournaments between August and September that cumulatively make up the Fed-Ex Cup.
Australia's Day is currently ranked No.1 in the world and No.2 in the Fed-Ex Cup standings after Patrick Reed's victory at The Barclays
in Farmingdale, New York, last weekend.
But Day is five shots off the pace in this week's event at TPC Boston where early pace-setters James Hahn and Ryan Moore scored opening rounds of 65 to tie for the lead at six-under-par.
England's Paul Casey, Argentina's Fabian Gomez, Colombia's Jhonattan Vegas and Brian Stuard of the US are all tied for second on five-under-par.
Elsewhere, Dustin Johnson, Jordan Spieth and British Open winner Henrik Stenson sit three shots adrift of the leaders on three-under-par.
No. 1 in the Fed-Ex rankings, Reed, is also on three-under-par as is Olympic champion Justin Rose.
The field will be cut down from the leading 100 players in the Fed-Ex Cup rankings after this weekend's play to the top 70 for next weekend's BMW Championship at Crooked Stick Golf Club in Carmel, Indiana.
The top 30 players will then compete for the overall Fed-Ex Cup title at the Tour Championship held at the East Lake Golf Club in Atlanta, Georgia, between September 22 and 25.