Bjørn -
Scottish weather is extremely unpredictable at any time of year - even the meteorological service admit this - but if I really had to choose, I would go for late May or late September. But the weather is such that we could get a week of bad weather, or a week of sunshine, or anything in between, at any time of year. In March and April this year we had many warm sunny days; we also had snow on the ground several times. May, June and July have been almost total washouts but, statistically, May and September give us a good chance.
But I suppose what I'm saying is, if it goes ahead, I'll be responsible for all the necessary arrangements but I won't be responsible for the weather!