Labour and the Conservatives are going head-to-head over their respective economic plans on Monday (6 April), the official start of the tax year.
The Tories have dubbed the day "money back Monday," with Prime Minister David Cameron due to speak alongside Chancellor George Osborne at a campaign event in the southwest of England.
Cameron will say: "Today is a big day for our country. It's 'money-back Monday' – a day when, quite simply, hardworking taxpayers get to keep more of their own cash.
"Low taxes go to the heart of what I believe as a Conservative. For me, there's the economic case: that, in a competitive world, we need to incentivise work by making it pay.
"There's the progressive case: that the best way to help with living standards is to let people keep more of the money they earn.
"But above all, there is the moral case: that it is wrong – frankly immoral – for government to spend money like it grows on trees."
But according to Labour, average families are £1,100 a year worse off since the Conservative-led coalition came to power in 2010.
Labour's shadow chancellor Ed Balls will unveil his party's latest election poster in Yorkshire on Monday, and will say: "Families are £1,100 a year worse off on average – the true cost of this Tory chancellor, as our poster shows.
"These are independent figures from the Institute for Fiscal Studies, which take into account all of the changes introduced by David Cameron's government from May 2010 to today – including the changes to the personal allowance.
"And while millions are paying more, millionaires are paying less. Two years ago today they gave the top 1% of earners in our country a £3bn a year tax cut."
Both parties say their rivals secretly plan to raise taxes if they win the general election.
Labour claims the Conservatives will have to increase VAT to make the maths add up, while the Tories claim Labour will be forced to increase National Insurance contributions to cover their spending plans.
New tax-year changes
Also on Monday, sweeping new pension changes came into effect, giving savers more control over their money.
The reforms mean that those with a defined contribution pension pot can withdraw as much of it as they like when they reach the age of 55.
Tax changes will also make it easier to pass pension savings to descendants, and some of those retiring will no longer have to buy an annuity to provide a fixed, regular income.
Other changes coming into effect include a higher limit on ISA savings, the Marriage Tax Allowance, the abolition of employer NI on under 21-year-olds, a 2.5% increase in the state pension, and the scrapping of the 10p tax band on savings.