This calendar was created for Isabella of Castile in the 1480s.
Remember medieval calendars are perpetual.
The Roman numerals or golden numbers on the left hand side of the page tells you when the new moon will appear and logically when 14 days later the full moon will appear. The numbers are from 1 to 19 and represent the metonic cycle.
Thus you need to know where you are in the 19 year cycle to work out which of the 19 numbers is the new moon fo 4th year you happen to find yourself in.
The metonic cycle basically works on the premise that across a period of 19 years there are approximately 235 lunar cycles after which the cycle will repeat itself on the same day ie the moon will be in the same place in the sky with the same stars. The cycle was discovered by the Ancient Greeks.
The golden number of any calendar year (Julian or Gregorian) can be worked out by dividing the year by 19. Now add 1 to the remainder, and that number is the golden number for the year.
2021 divided by 19 = 106 remainder 7
7 + 1 = 8. So in 2021 the Roman number 8 will provide the day in the metonic cycle on which a new moon appears. Of course you’d need to know which days fell where within the cycle to do the calculation. Then it’s a question of counting on 14 days to calculate the full moon. This was important for working out when Easter would fall in any given year (the first full moon after the Spring equinox.) Across the metonic cycle Easter could only happen on 19 specific dates and if you knew where you were within the cycle you could calculate this.
2021 is the eighth year of the metonic cycle but applying the medieval perpetual calculator to work out the date of the new moon in 2021 won’t work because of the drift in the Julian calendar – when we changed to the Gregorian calendar we lost 10 days! The first new moon of 2021 falls at about 5.00am on January 13th and is apparently a wolf moon whereas the Golden Numeral method of calculation states that the new moon falls on the 6th…which is clearly not correct!
The calendar page for January often depicts Aquarius- usually a bloke pouring water from a downturned jug. There may sometimes be an image of Janus – the two-headed god after whom January is named looking towards the future and back towards the past.
The main agricultural/seasonal illustration is often a winter scene or someone warming themselves by the fire as above.
The British Library has a lot of useful information on medieval calendars.