The engaged singers applied for their marriage license Tuesday in Oklahoma ... according to the Johnston County Court Clerk. The clock's now ticking for Blake and Gwen to walk down the aisle -- in Oklahoma folks have 10 days from the date they file for a marriage license to tie the knot.

Sources with knowledge tell TMZ ... Gwen and Blake are set to marry Saturday at Blake's Oklahoma ranch.

We're told the couple has an entire weekend of activities and celebrations planned, but they're staying a bit flexible due to the weather forecast. If mother nature cooperates, the ceremony will be held in an outdoor chapel on the ranch.

Our sources say Gwen, Blake and their kids are already in OK ... we're told they flew out from Los Angeles last Friday after Blake finished filming for "The Voice."

We're told the couple is sending their private plane from Oklahoma to L.A. Thursday to pick up Gwen's immediate family and whisk them to the heartland.

While the wedding is going down on a holiday weekend, sources say it will not be very big ... we're told only close friends and family will be on hand.