Using the original short story as part of my argument here, since Ennis Del Mar was still working at the Stoutamire Ranch when he called Lureen and the fact that because as he got older, Ennis seems to have become more stable in his work ethic. That's partly why Ennis said that meeting in May was a trade off for August which might have been in Jack's and Ennis's original plans to meet both in May and in August.
IMO, Ennis does not seem to be the type to suddenly drop something and rush off. And, the only time that he suddenly did that was at the reunion in 1967. He was only supposed to take one day off in June 1967, the 24th, from his highway crew job to meet Jack. Ennis was working on a ranch on weekends at the time, too.
Oh, Ennis did get easy-to-quit jobs that he could leave if Jack agreed to meet him somewhere; but, those meetings still had to be planned ahead of time.