I have visited the UK (London, Reading, Swindon) several times so far. During my last trip to Swindon, I noticed that many restaurants are closed on Mondays.
Is this practice specific to Swindon or valid all over the UK?
Edit: Since Monday is not a common holiday in Turkey, it is not common for me either.
Let's use a bit of real data to give you a full answer. I've checked Yelp to find the opening times of restaurants in London, Reading and Swindon (thanks to @ZachLipton for the idea!). The findings are summarised below:
| City | Open on Fri 7PM | Open on Mon 7PM | % closed on Mon |
|---------|-----------------|-----------------|-----------------|
| London | 8073 | 7326 | 9.3% |
| Reading | 130 | 125 | 3.8% |
| Swindon | 41 | 36 | 12.2% |
So the answer is no, it is not too common for a restaurant to be closed on Mondays in the UK.
We can also compare our findings with equivalent statistics for the Turkish cities of Istanbul, Ankara, Izmir:
| City | Open on Fri 7PM | Open on Mon 7PM | % closed on Mon |
|----------|-----------------|-----------------|-----------------|
| Istanbul | 3174 | 3119 | 1.7 |
| Ankara | 481 | 473 | 1.7 |
| Izmir | 270 | 266 | 1.5 |
The data above confirms your observation that it is very uncommon for Turkish restaurants to be closed on Monday.
It is common but not universal. Chain restaurants are usually open all days, smaller restaurants may take a day off during the week and Monday is a sensible choice because more people eat out over the weekend.
Other businesses, particularly small businesses, may do the same if most of their trade is weekend shoppers.
If you are wanting to visit a particular restaurant, or are staying in an area with only a couple of places to eat out available, it would certainly be worth checking out in advance. In most areas, though, there will be other restaurants around that are open so it is only a minor inconvienience.
I would say that most restaurants in the UK are open seven days a week. However, if a restaurant is going to close for a day, that day will almost always be Monday.
The same holds for shops and museums – and any other business that does most of its trade at the weekend. (And, if such a business is closed for two days, they'll likely be Monday and Tuesday.) This isn't unique to the UK: I've seen the same thing in the US, for example.
It's noted in the comments below the question that public holidays in the UK are commonly on Mondays. In England, Wales and Northern Ireland, the day after Easter Sunday, the first and last Mondays in May and the last Monday in August; in Scotland, the first and last Mondays in May and the first Monday in August. However, I don't think that's an issue: restaurants and shops aren't legally obliged to close on these days and, since they're holidays, they're likely to be very good for business. I'd certainly expect a museum that usually closes on Mondays to open on holiday Mondays.
From my experience this is more common in smaller towns and possibly in the North. I live in the South (about 50 miles North of London) in a fairly large town and it's not terribly common here. I know of a few places which are closed on Mondays but it tends to be smaller and often take-away places (Chinese, chip shops, etc.). On the other hand, I have been on holiday to smaller towns (one springs to mind in the Peak District) where almost everything was closed on a Monday night except for one Indian take-away place. It may be a regional thing or may be more due to the size of town, or a bit of both.
So I would say that it's somewhat common, but far from universal, and may be more common in some regions than others.
