date - range

Anybody a solution for the next problem:

people can subscribe to a service for 1 or more days (upto a max. of 365).

So in the database is stored: first_date and last_date

To select which people are subscribed for a certain date (e.g. today) we use
a select like

select   ....... where first_date <= today and last_date >= today

Whatever index we create system always does a sequential scan (which I can
understand).

Has someone a smarter solution?

All suggestions will be welcomed.