No penalty. You get rewarded for your accepted share over the last n shares when the block is found.
Yours shares on the weekend will be counted if a block is found while you are not mining on Monday.
You not mining all daytime Friday will reflect if a block is found on the weekend with its 5n look back (currently ~4days 9hrs.).
It averages itself out to an effective hashrate "Your N Avg" for mining.
I am not yet entirely convinced that it averages itself out and am working on a calculation. I believe that there is a disadvantage for sporadic miners and newbies, anyone who has not filled the 5Nd window with a flatline hashrate. Once I have my results I will publish here.