by rmsgrey
The game board has a list of month just below the objectives space, which tells you how many objectives you need to complete that month (the number of stars below the month name). If memory serves, that's three objectives for May.
You should have added several objectives to the game during the earlier months. All of them that haven't been destroyed yet are available for completion during the game.
If, at any time during the game, you have at least three objectives that are currently complete, and all your mandatory objectives are among those currently complete objectives, then the game ends immediately (or almost - there are some corner cases where a bit of tidying up happens - for example, if the Medic is in the same city as the last cubes of a disease when you find the cure for that disease, and finding that cure completes an objective and means you now win, then the Medic's ability will still clear those cubes and the disease will be eradicated) and you win.
You never have to choose which three objectives will count for that game - though you may choose to focus on some more than others - all that matters is that you complete the mandatory ones and at least enough others to get to three.