A young lad goes to the pharmacy to purchase some condoms

A young lad goes to the pharmacy to purchase some condoms. He's got a date
this weekend and wants to be prepared..."Just in case." He looks around and
doesn't quite know what to make of all the various packages, and finally asks a
clerk for some help.
"Sure, I can help you", says the clerk. "What do you want to know?"
"Well," replies the lad, "I have a date this weekend, and want to be
prepared...just in case, you know? But I've never bought condoms before and I
don't quite know what I should be getting."
"Well," says the clerk, "this here is a 3-pack of condoms. That should do
you just fine."
"Why 3?", says the lad.
"Well, one for Friday, one for Saturday, and an extra just in case one
breaks.", replies the clerk.
"Hmm....what's this 7-pack for?", says the lad.
"That's for if the relationship goes past the weekend. One for every day of
the week." says the clerk.
"Oh my, then what's this 12-pack for?", says the lad.
"Oh. You don't need that.", says the clerk.
"Well why not?", says the lad. "Seems to be more economical."
"Well," says the clerk, "That's for the married men. One for January, one
for February, one for March...."