help with a monthly rollup

I need to rollup data by month.
There are a couple ways to do this.

I have a date field. So I can rollup with a to_char(mydate,'YYYYMM')
this requires a where clause as followings
where to_char(mydate,'YYYYMM') = pSomeMonth;

This means I need a function based index to support this. We have alot of
data, so where possible, I want to avoid adding more indexes. Is there a way
to calculate what the last
day of the month is?

so I can use a between as follows
where myDate between to_date(01-MON-YYYY') and

Since the last day of the month can be

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