de-mob = demobilisation. As in when soldiers leave the army (can apply to anyone really).

Blokes, after the war were issued 'de-mob suits' etc.

If you are said to be 'de-mob happy' you are probably lightheaded due to the impending release from duty.

Not quite Christmassy yet, PM - hope with the next few days holiday I will get into the spirit of things! :P