Getting Error - ORA-01858: a non-numeric character was found where a numeric was expected

The error you are getting is either because you are doing TO_DATE on a column that's already a date, and you're using a format mask that is different to your nls_date_format parameter[1] or because the event_occurrence column contains data that isn't a number.

You need to a) correct your query so that it's not using TO_DATE on the date column, and b) correct your data, if event_occurrence is supposed to be just numbers.

And fix the datatype of that column to make sure you can only store numbers.

[1] What Oracle does when you do: TO_DATE(date_column, non_default_format_mask) is: TO_DATE(TO_CHAR(date_column, nls_date_format), non_default_format_mask)

Generally, the default nls_date_format parameter is set to dd-MON-yy, so in your query, what is likely to be happening is your date column is converted to a string in the format dd-MON-yy, and you're then turning it back to a date using the format MMDD. The string is not in this format, so you get an error.

This error can come not only because of the Date conversions

This error can come when we try to pass date whereas varchar is expected
or
when we try to pass varchar whereas date is expected.

Use to_char(sysdate,'YYYY-MM-DD') when varchar is expected