Mik,
This is the sort of problems with trying to establish fixed dates for parking here.
I noticed the BDO have posted this
>Caer Abiri Gorsedd and dates
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Greywolf proposed that the BDO should host gatherings of the Gorsedd of Bards of Caer Abiri at Avebury on the nearest Saturday to the four festivals of Candlemas, May Day, Lammas and Hallowe'en by the old Julian calendar. The provisional dates are as follows: -

Hallowe'en 2003 November 8th
Candlemas 2004 February 7th
May Day 2004 May 9th
Lammas 2004 August 7th
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http://www.awenlist.org/index.php

which is a week After the tradtional dates for these festivals.
PeteG