Observation of Mercury14/09/2021 @ 20:00 - 21:30 - 14th September. This is one of the best days of the year to observe the planet Mercury by the end of the afternoon. Observatório do Lago Alqueva14 September, 2021 When 14/09/2021 20:00 - 21:30 Add To Calendar Download ICS Google Calendar iCalendar Office 365 Outlook Live This is one of the best days of the year to observe the planet Mercury by the end of the afternoon.