Some 620 people in the UK filed their tax returns online on Christmas Day, according to HM Revenue and Customs.
The 224 accountants and 396 individuals took time out on the national holiday to complete their self-assessment - over a month ahead of the deadline.
A Revenue advertising campaign is stressing that 31 January is the deadline for filing returns for the 2007/08 tax year.
Submissions must be made online as paper returns face a late payment fine.
The £100 penalty is being handed out to those who filed paper returns after 31 October.
This is the first time that two deadlines for the two different formats have been imposed.
Registration
Self-assessment involves completing a tax return in order to tell the HMRC about income and capital gains - profits on the sale of certain assets - or to claim tax allowances or reliefs against your tax bill.
This is required primarily from the self-employed or those who have income from several sources.
A growing number of people have been using the online system for their self-assessment tax returns, although the system has had its problems in previous years.
Last year, the deadline was extended after thousands of people found they were unable to log on to the Revenue website on deadline day.
The Revenue said that the deadline was fixed for this year and those who filed late would be fined £100.
Anyone registering for the first time should be aware of one potential delay.
Once registered, you will be sent a pin code by post which can take up to seven days to arrive.
"Make sure you register by 21 January, to allow enough time for the pin to arrive and for you to file your return online," a spokesman said.
In January and February, the Revenue will be writing to individuals with their new tax codes for the 2009/2010 tax year.
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