First closing date comes into play when one company launches a takeover bid for another, given it has to observe rules as laid down by the City's Panel on Takeovers and mergers ( POTAM ).This set of rules can be very complicated. For example if a company says it's going to bid for another company, the bidder has 28 days to formally present 'the Offer Document'.
After dispatching this document to the shareholders of the target company (the one receiving the bid), it has to leave the Offer open for at least 21 days.
The end of this 21 day period is deemed to be 'the First Closing Date'.