If you want to use a variable in an expression, define it in the variables section found at the very bottom of the assets pane, and the definition of the varaible can be blank (set by a script, Vizlib, etc), or itself an expression.
If you want to use a variable in the load editor, just state it by using a statement like LET vmonth = something; (Or use SET if you want the expression to not be calculated until called by $() expansion)
You can also do this directly in the field by using a complex seeded IF statement, like:
If([field] = 'JAN', do january thing, If([field] = 'FEB', do february thing, If([field] = 'MAR', do march thing, else)))
Where the syntax is effectively If(expression, THEN, ELSE)
