If you have a database you could use a table or create a simple XML to use as a secondary datasource.
Two columns
Number Name
01 January
02 February
Etc...
Then in Open Rules parse the date. Since the date is yyyy-mm-dd we need to get just the month from the string and then use it to look up the correct value from the secondary data source list.
Set your drop down to Name[number = substring-before(substring-after(today(), "-"), "-")]