I dont think fascinators are allowed in the Royal Enclosure anymore unless they have a base of 10cm or more! Assuming you are going to be in the Grandstand these are the guidelines...
Ladies
Ladies within the main Grandstand enclosure are encouraged to dress in a manner as befits a formal occasion.
Ladies are kindly asked to take particular note of the following:
A hat, headpiece or fascinator should be worn at all times
Strapless or sheer strap dresses and tops are not permitted
Trousers must be full length and worn with a top that adheres to the guidelines above (i.e. strapless or sheer strap tops are not permitted)
Jackets and pashminas may be worn but dresses and tops underneath should still comply with the Grandstand Admission dress code
Midriffs must be covered
Shorts are not permitted.