From the joint announcement on 14th April from Unite and BA;
The current strike ballot mandate given by cabin crew means industrial action must be announced by 15 April. Both parties accept that this would severely undermine any attempt at a genuine settlement. Accordingly, British Airways is prepared to grant an extension of 28 days to the Union.
15th April plus 28 days is Friday 13th May. Where do they get 9th May from?
Dave