I've been trying to use fitday regularly but I will sometimes forget a day. Is there anyway to enter foods eaten for the previous day?

Yes, at the top of the screen you should see something like
<< June 9 >>
if you click on the << or the >> you can move forward or back a day. Also if you click on June 9 it will show you a picture of the whole month, which you can chose a date to enter foods for.

Nothing to forgive

It's easy - when you're on your main page, you can see the date at the top of the page. There are arrows to the left and right of the date. Click on the left arrows to go to previous days.

Hope that helped!

***edited to add: 2 helpful people at once ***

Thanks a lot. I try to keep a log of my own along w/ fitday but sometimes I lose track.