### Clue: Schedule this month's one that can't be changed (3,2,5)
### Answer: SET IN STONE 
 ### Explanation: Definition: that can't be changed
 SET(to schedule/to make arrangements for something to be done, as in "to set someone to do a task") + INST(abbrev. for "instant" indicating this/present month, as in "your letter of the 16th inst.") + ONE.