how to write a calendar with pl/sql?

But while trying to compile it , it is showing error:

CAL_STR1 := 'SUN';
*

ERROR at line 18:
ORA-06550: line 18, column 10:
PLS-00103: Encountered the symbol "=" when expecting one of the following:
constant exception <an identifier>
<a double-quoted delimited-identifier> table LONG_ double ref ...