Linux is case sensitive. It looks like you used an upper-case "If" instead of the correct "if" (all lower-case). Furthermore the syntax is not correct. It should be
if [ $(date +%a) = Mon ]
Also, please use code tags to embed the lines of code in your posts: they will make the code more readable. To put code tags, you can go in advanced mode (see the button "Go Advanced" below), select the text you want to enclose in code tags and press the "#" button. Alternatively you can explicitly write [CODE] and [/CODE] before and after the text respectively.