@sonyagreen,
"Was it anyhow connected with sanctity of Saturday that Jewish people observe?"
If both of your brain cells were to mull this over for a while and decide that a little research was required before you asked this question, you would have discovered that the Jewish Sabbath begins at sundown on Friday evening.
At that precise time, dear William was most probably playing conjugal "hide the sausage" with Kate, before dressing and going on to boogie the night away until well into the early hours of Saturday morning.
The only Jewish tradition that the Queen takes part in is the occasional scoffing of the breakfast dish of Yom Kipper, a Scottish fish recipe made from cured bloaters caught on the banks of the River Yom, in south Lanarkshire.