The month is a unit of time and is a multiple of an SI base unit with the symbol mo.
There are 100s of definitions of a month that have been established, reviewed and adopted over the course of history but the most common around the world today are the months in the Julian and Gregorian calendars.
There are 12 months in the Julian and Gregorian year; averaging 30.4368 days in each. They are labelled January, February, March, April, May, June, July, August, September, October, November and December.
There is a famous rhyme to help remember how many days are in each month that goes “Thirty days has September, April, June and November – all the rest have thirty-one. Except February which has 28 or 29 in a leap year.”
Many cultures use the moon’s cycle (the lunar cycle) to define which month they are in.
The day is a unit of time and is an SI-derived unit with the symbol d.
On Earth, it is defined as 86,400 seconds and is approximately the time it takes for the earth to complete a full rotation around its axis. In the earlier days, this was measured by waiting for a cast shadow to match a template drawn from the previous day's shadow.
There are 365 days in a year and, on average, 30.42 days in a month.
The unit day has many different variants; depending on what is used to measure the Earth's rotation. In a sidereal day (a rotation with respect to a distant star or constellation, not the sun), there is actually 4 minutes less than 24 hours in a cycle.