Execute SQL query ever end of month

Hi,
I am using the Informix database with dbaccess data_base_name. Could you please let me know how to execute a SQL query every month end without modification of the query? For example every end of month I have a dozen of SQL queries to be executed and I have to send the results to my colleagues but I have to edit each of them to modify the period from Day 1 to Day 30 if it’s November or from Day 1 to Day 31 if it’s December, etc.
How should I write these fields in my SQL script?
Thanks.
