This week, we read the passage in Leviticus that deals with our Shabbat and festivals.

Shabbat is our day of rest, and the festivals are intended as days of joy and rejoicing.

We are not supposed to be sad on Shabbat. We are commanded to be happy on our festivals.

Being currently engaged in counting the Omer, we recognise the importance of the chagim in our lives.

They have to be at the right time of year, and this year are later than usual, owing to the leap year, which keeps the lunar cycle in line with the solar.

Shabbat differs from the festivals in an essential manner. Shabbat happens every week and can be kept track of very simply by counting to seven.

It is the simplest of all our holy days, and the surest. There can be no dispute about when it arrives.

Shabbat, and its interval, were set permanently by God.

This is not so with festivals. They are proclaimed, and depend on an accurate mensuration of the calendar.

In ancient times, great care was taken to proclaim the New Moon at the correct time, and witnesses of the sliver of new moon-silver in the sky were cross-examined.

Indeed, the date of Shavuot was the subject of dispute between us Rabbinites and the Sadducees and others.

The Torah says we start counting the Omer “from the day after the sabbath”, with the rabbis understanding that this means the second day of Pesach. The Sadducees took this as the day after Shabbat literally – making Shavuot always on a Sunday.

The essence of this (debatable) human agency in declaring the dates of our festivals is embedded in Chapter 24, verse 4: “These are the festivals of the Lord, called holy, those which you shall call in their seasons.”

For Shabbat, God took responsibility for the arrangements.

For the chagim, our festivals, we are given responsibility.

Even if we make mistakes and our calendar is inaccurate or we are a day or two out, merely by declaring the calendar we create the date of the festivals.

That is our big responsibility.

• Zvi Solomons is rabbi of the Jewish community of Berkshire. See www.JCoB.org