A really indepth look at the centre of Heathenism in Sweden at Uppsala, I recommend clicking the link and waiting for the full pdf file to load so that you can see the entire text & all the pictures too. Well worth the read.

Here's the abstract
The heathen Great Midwinter Sacrifice and the ‘royal’ mounds at Old

by Göran Henriksson

According to the established interpretation, the heathen Great Midwinter Sacrifice at Old Uppsala, in Uppland province, took place every ninth year. In our modern way of counting, this means every eighth year. The starting date was determined by the full moon that occurred between 21 January and 19 February in the Julian calendar. After the introduction of Christianity, the
tradition with a great assembly at Old Uppsala at midwinter was continued by the Disting, which consisted of an assembly and a market. By combining historical data and calculations of the dates of the full moons within the Disting period, it has been possible to establish the exact years of the
eight-year cycle. One such year was AD 852 the same year as St Ansgar's second missionary journey to Birka, the oldest Swedish town.
At the end of the 17th century, the farmers of Uppland were still using the so-called rule of King Aun, according to which the phases of the moon in the Julian calendar fell one day earlier after 304 years. Such displacements in the eight-year cycle took place in 1692, 1388, 1084, 780, and 476. The semi-legendary king Aun is considered to have reigned about AD 450-500 and to have been buried at Old Uppsala. The three ‘royal’ burial mounds there have been dated to AD 450-550. These mounds are oriented in such a way that they could have been used to regulate the sacrificial calendar.