There are three major religions in Israel: Judaism, Christianity and Islam. Jerusalem is a holy city for all three of them.
While some people, maybe many, don't follow their religious beliefs strictly, there is a considerable population of ultra-orthodox Jews. You can easily notice them in Jerusalem, since they wear in a very traditional way.
Even during a short visit of in Israel, even without visiting any church, synagogue, or mosque, you can still feel that Israel is a highly highly religious country -- from the way they observe the holy holidays. Your activities do get affected.
During a week, Shabbat is from the sunset time on Friday to Saturday evening. In Old Jaffa, I happened to run into a park where there was a stage performance on Friday afternoon. It's traditional Jewish music for greeting and celebrating the arrival of Shabbat.
In Shabbat, there is no bus or train services. Most stores are closed, only a few cafes and restaurants are open.
This street is a light rail only street. It's Saturday, no light rail service. People feel free to walk between tracks.
During my visit there was a major holiday, the Yom Kippur. From the eve to the night of Yom Kippur, the closure is of a much larger scale. Even the international airport is closed. No vehicles are allowed to run on the streets, except ambulances. All stores close, including restaurants. People fast for 25 hours.
In the morning of Yom Kippur, Jews go to the Western Wall to pray and chant.
Friday evening in Jerusalem is quite special. People dress up very formally and go to the Western Wall. It's a very serious ceremony!
The last place I visited is Haifa, a city built on the steep slope of a mountain. I hiked uphill for half an hour to attend a guided tour, just to find out that it was cancelled because it was the eve of Sukkot, a week-long holiday. The next day was the first day of Sukkot, it's like Shabbat, no bus during the day time. Fortunately I was able to find a cafe that was still open for lunch.
The day before I went to Haifa, I found many street venders were selling interesting things, as shown in these photos. Eventually I found that they are only for Sukkot.
There is a famous Arab market. But when I got there, all restaurants were closed. Later my BnB host told me that the market is mostly run by Arab Christians, who take Sundays as their rest days. So, in an ordinary week, Jews take a rest on Saturday, Christians rest on Sunday, while Muslims rest on Friday.
For solo tourists like me, definitely it requires lots of planning in such a religious country. My 10-day stay happened to include two Shabbat, one Yom Kippur, and the first day of Sukkot. On a Saturday I hiked 11km for a roundtrip of visiting the Israel Museum in Jerusalem. The last night, I had to carry my backpacks and hike 45 minutes to get to the railway station. I think I became stronger after my trip!