Richard Mellish wrote
"And the July dates surprised me,"

Hmm. I think that the 'July' is not as important as conveying the three days of the verse that runs:-

They hoisted their sails on Monenday morn,
With all the speed they may;
And they have landed in Noroway
Upon a Wodensday

... and I think we must be careful about taking what is clearly intended as a bit of fun too seriously.