@PanchoSimon, definitely best to ignore kit instruction timings! You'll see many mentions on this forum that these instructions are always hopelessly optimistic!
At 18-20 degC your brew will take about 2 weeks to fully ferment out in FV. So, think best hold fire on dry-hopping for a few more days yet.
Others may correct me, but I believe you generally want to wait until fermentation has slowed right down to a crawl. (I usually aim to dry-hop when brew has hit the last 2-3 SG points above target FG. This is in the knowledge that the brew has, at that stage, around 5-7 more days before fermentation is complete.)
This reason for this choice of timing is that the dry-hopping process is really there to deliver aroma only to the finished brew. (Delivery of hoppy bitterness comes from earlier hop boil - whether in brew kettle during an AG process, or during Festival's kit extract manufacture).
If you dry-hop while fermentation is too active, then the volatiles you really want to be retained for aroma in your brew will be stripped out by the escaping CO2 bubbling through your airlock. (Aroma that you can smell in the escaping CO2 will naturally no longer be aroma retained in your beer!)
You cannot, within reason, start dry-hopping too late. The time your brew spends on yeast in FV prior to dry-hopping is certainly never wasted by the yeast, (even if fermentation per SG appears complete).
It allows the yeast to clean up and condition at a faster rate than it's ever going to be able to in bottle (there's more yeast available to do the job). And you'll need to allow around 2 weeks for that hugely important bottle conditioning process (which the instructions never mention!)
You can start dry-hopping too early though (or effectively, for too long). Beyond 10 days on dry-hops and you'll start to notice grassy tones developing (a bit like avoiding an over-stewed tea or coffee).
So, the optimum really is to aim to dry-hop for just the last 5-7 days your brew spends in FV before bottling.
If started when you've hit those last few SG points, then you can reasonably expect that the fermentation finish/bottling stage will coincide with optimal finish time for dry-hopping.
All of that said, a few days here or there are very unlikely to make a perceptible difference. Certainly don't lose sleep over it. You'll still create a great beer, I'm sure!
From personal experience, and I've done about 15 Festival/MJ kits these last 12 months, Festival kits have proven time and time again to be hugely reliable. Their Pride of London Porter - in particular - is excellent!
BTW regarding use of the muslin filter bag they provide. Each to their own, but rather than bag the pellets, I always dry-hop loose and then syphon through the mesh of a stainless steel hop spider when racking into my bottling bucket from the FV. (See earlier post below...)
https://www.thehomebrewforum.co.uk/threads/bottle-wand-very-slow.93165/post-1016949
