I suppose it depends on your niche and how much coverage you site has, but yes i have experienced a similar drop
How about previous days.......? It looks like it was down before "Monday" and start to move up then moving down again...
Hey man don't worry do write some articles and submit them in social networks with your site links . Sometime our niche don't get expected search queries and we start getting worried .
i have a site averaging 2,000 - 2,500 hits daily, Fridays and Saturdays are always low days for me, around 1,600 - 1,800 but like what has already been said, this will vary depending on niche. maybe your SERPS are dropping but you would need a little more time before you could say for sure, in the meantime continue to build quality backlinks, that will help for sure GRG, what is FSB?
Apparently this is one of the busiest weekends where I live for travel so it may be like that across the board.