There are several small problems.
Firstly, the To_Char function wants the format_mask as a string, and I wonder if you really want the ampersand to be a part of the date.
Secondly, you have a semicolon in the middle of the second row that terminates the query. I also recommend to always use a format_mask here to, to avoid misunderstandings.
So try:
SELECT TO_CHAR(promo_begin_date,'dd/month')
FROM promotions
WHERE promo_begin_date IN (TO_DATE(JUN 01 98,'MON DD YY'),TO_DATE(JUL 01 98,'MON DD YY'));
More info on format_masks
here[
^]