Notes
Field: Today__c = 12/12/2021
Let's create another field with the data type Formula;
Data Type: Formula
Formula Return Type: Text
Field Label: Which month?
Field Name (Auto): Which_month__c
Formula:
CASE(
MONTH(Today__c),
1, "January",
2, "February",
3, "March",
4, "April",
5, "May",
6, "June",
7, "July",
8, "August",
9, "September",
10, "October",
11, "November",
"December"
)
MONTH(Today__c) = 12
Output: Which_month__c = December