What to do when rent is late and when is it late?

[Mark Malevskis]

Hello, 

I have just purchased a six unit last month. 

Rents were due on the 1st. I have installed a rent drop box on the first floor and passed out a letter to all of the tenants to deposit checks in their. 

I check the box yesterday and 0 checks were there. 

According to their rental contract, rent is due on the 1st but they have a week to pay before late charge kicks in. 

What do you think I do at this point? 

Should I wait for the rent to come in by the 7th or should I go in there and individually ask each tenant why they haven't made a payment?

Any ides would be great, 

Thank you!