The apostrophe S has two uses:

1. to connote ownership
2. as a contraction

correct examples:

• This is Martha’s machete.
• Martha’s waiting for George.

Never use an apostrophe to show plural.

examples using an apostrophe to show plural:

• On Tuesday’s, the special is spam, spam, spam, eggs, and spam.
• There were three CD’s on the table.

corrections:

• On Tuesdays, the special is spam, spam, spam, eggs, and spam.
• There were three CDs on the table.