Quick way to fill up blank cells with preceding data

The quickest way to fill blanks in a data set to match the values in the rows above is to

1) Select the column(s) of interest that you want to fill - in your case, B and D though you can select C as well
2) Press Ctrl-G and choose Special... then choose "Blanks" and press OK
3) Type =, press the up arrow key once, and press Ctrl-Enter.
4) Select column B, copy, paste values
5) Select column D, copy, paste Values
6) fix your formatting

To fix the February issue, you need to use a conditional in your date formula: in C2, use this

=IFERROR(IF([@Day]="","",DATE($H$2,MONTH(C1)+IF([@Day]<B1,1,0),B2)),DATE($H$2,$I$2,[@Day]))

Though once you fill B, there should be no need for the check for ""

=IFERROR(DATE($H$2,MONTH(C1)+IF([@Day]<B1,1,0),B2),DATE($H$2,$I$2,[@Day]))