The short answer is: every Sunday. Why?
As far as I can make out, Jesus gave only one instruction about worship to his disciples: "Do this in remembrance of me". The sharing of the bread and the wine at the Last Supper occurs in all three Synoptic Gospels and it is referred to again by St Paul, as well as being hinted at in various passages in the Fourth Gospel.
In Acts 2:46 we read: "And day by day, [the disciples were] attending the temple together and breaking bread in their homes." Sharing of bread (and wine) was clearly the way these first Christians distinguished themselves from their Jewish and pagan neighbours. At that stage, of course there were no churches to worship in, so it had to be done in their homes. Interestingly, it seems to have been a daily event, perhaps showing us that they interpreted the phrase "Give us this day our daily bread" as being about the Eucharist.
Not long after this, as the Church spread, Sunday became the day for worship and again the Eucharist was the normal form of worship. Throughout the Middle Ages, the weekly Mass (Eucharist) was the only form of service in most parish churches. The majority of people received Communion only once or twice a year, however. This was one of the abuses the Reformers, both in Britain and on the Continent, wanted to correct. They wished to restore the practice of the early Church, which was "the Lord's people at the Lord's table on the Lord's day". Unfortunately, old habits die hard and it was not until the twentieth century that weekly Communion began to be the norm in Anglican churches. In our generation is has also become more widespread in other churches, following the lead of the Taizé and Iona Communities in the Reformed tradition, for instance.
Another welcome development is that the Communion services of many churches are now remarkably similar. At the same time it is increasingly common for the different churches to invite people from other traditions to receive Communion with them.
Weekly Communion, the practice of the earliest Christians, is no longer the preserve of the few but a sign that "all who love the Lord Jesus are welcome at his table", which is where we belong.
Peter