Auto populating cells with data

I have a spreadsheet which involves a lot of data entry. What I am trying to do is have as much info automatically entered as possible.
Some of the columns include: date, week in year, period, and period/week
If I enter the date as say Jan 6th, how to I get the cell in the "period" column to know that since jan 6 is between jan 1-jan28 that it falls in period 1 week 1?
This is really confusing to explain what I'm trying to do, but please help. Thanks

All I can think of is using an if function: if(a1=date between jan 1 and jan 28, then "period 1"
Except I don't know how to write that