Suppose a lease begins on February 26th, and the monthly rent is £10000. There would be 3 days of February included, but how would the rent payments for these days be calculated? February only has 28 days: Would it be (10000/28) × 3 [=1071.4], or (10000 × 12/365) × 3[=986.3]?
The method of calculation may depend on the nature of the lease, and where the property is located.
If this is a rental of a place to live in (a "dwelling-house") in England & Wales with an Assured Shortland Tenancy, then the answer is: the calculation doesn't happen, because a tenancy with a monthly period runs from whatever date of the month it starts on.
So in this case, every month's rent is £10,000, and it is due on 26th of every month.
There is, however, a provision for if the tenancy ends part-way through a month. Section 21C of the Housing Act 1988 states that is calculated based on the number of days in the final rental period - hence it will indeed vary depending on when that occurs.
As the contract says
The normal usage of the word “month” is a period of time from a given date in one month to the day before that same day in the following month. So, for your example of a lease starting on 26 February, the first month’s rent covers the month from 26 February to 25 March (28 days) and the second from 26 March to 25 April (31 days).
The alternative reading is the named “month” when it refers to the months of the calendar (January, February etc.). If that is so and the contract does not specify a pro-rata regime, then the whole figure is payable for any part of the month. If the contract does make provision for pro-rata then it should define how that happens. If it doesn’t then the normal interpretation would be whatever fraction of the days of the particular month meaning that, yes, each day in February is more expensive than each day in March.