Purchasing a single day ticket, for any day of the event, will only grant access on the day you have purchased.
A 3 day ticket will grant access from Friday - Sunday
A 4 day ticket will grant access from Thursday - Sunday
Make sure to read the differences between Grandstands and Premium Grandstands below:
https://www.silverstone.co.uk/events/formula-1-british-grand-prix/tickets/grandstands