The ministerial dinner that announces the near close of the parliamentary session. Sir Robert Preston, M.P. for Dover, first invited his friend George Rose (Secretary of the Treasury) and an elder brother of the Trinity House to dine with him at his fishing cottage on the banks of Dagenham Lake. This was at the close of the session. Rose on one occasion proposed that Mr. Pitt, their mutual friend, should be asked to join them; this was done, and Pitt promised to repeat his visit the year following, when other members swelled the party. This went on for several years, when Pitt suggested that the muster should be in future nearer town, and Greenwich was selected. Lord Camden next advised that each man should pay his quota. The dinner became an annual feast, and was until lately (1892) a matter of course. The time of meeting was Trinity Monday, or as near Trinity Monday as circumstances would allow, and therefore was near the close of the session.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894