Yes I’ve signed all the necessary papers and if they didn’t send the email, my employment contract would have still been valid.

This is extremely stingy from their side. They already signed the contract, you are new employee, they definitely don't want to start with the right foot if they want to change a signed contract because they don't want to pay one day of holiday more! You might want to keep looking because there will be more issues down the line.

@Cris: this might not be stingy but an administrative problem. They can't do paperwork on April 1st and back dating things on April 2 may be difficult and even illegal. The easiest way to deal with this, would be to add a sign-on on bonus of one day's payment which seems more bother than it's worth.

@QuantumMan: would you have blinked an eye if the original contract would have said April 2nd from the start? Please consider that this just may have been a honest mistake that's hard to mitigate for administrative or legal reasons. Do you really want to start your new job with picking a fight over something so minor ?

@Hilmar, to any same person, docking a day's pay is a case of an employer picking a fight with their new employee, not the other way around. There's no more incendiary attitude conceivable than treating the employee's pay as something to be forfeit for administrative convenience, not the ridiculous pretense that the employer considers pay a minor issue and yet cannot arrange to pay the extra day.

@Hilmar They are not picking a fight. They screwed up. Honest mistake. Getting upset is the wrong move.

@Hilmar, in this case, I don’t believe I’m the one picking the fight. They made the mistake and they want me to take the downfall due to their mistake and not the employer. It doesn’t seem right but rather than acting on emotions, it will be in my best interest to act professionally and hopefully it’s a misunderstanding

When you signed the contract were you aware that April 1 was Easter Monday (bank holiday)? If Monday is a holiday would you have just been as happy to go to work in an empty (or semi-empty) building? You haven't even begun working for them, do you really care that much over one day's holiday/pay? Do you think they chose April 1st (a Monday) because they wanted to save money?

This brings me back. My second full-time job's start date was on President's Day. I showed up, and my my boss and coworkers weren't there.

@DJClayworth "They screwed up. Honest mistake." - Would you also say that if the company reached out with "Hey, I know we already signed the contract, but we actually intended to offer you a 10% lower wage. Whoops, our bad. Please sign this new contract with the lower rate." ?

An interesting tangent to this question: given that (presumably) nobody from HR or legal will be working until 2 April, does any of this matter? By the time they learn of your acceptance or non-acceptance of the variation, you will have started, regardless of whether you choose to accept it or not. I would be tempted to just say nothing and see if anyone ever follows up.

So you think you've earned April 1st pay by ... ? Sure, going back on whats signed is pitiful, but you are starting on April 2nd ... They are just adjusting the contract to match reality. You could also ask if they'll let you start on March 30, after all, they also pay Saturday and Sunday to their employees.

@marcelm Of course not, they are completely different. Why would you think they are the same?

"Your mission, should you choose to accept it, is to work peaceably with fallible humans. Good luck. This comment will self-destruct in 5 moments." (cue Mission IMPOSSIBLE theme music)

@DonQuiKong they've earned that day's pay in exactly the same way the company's other staff are earning their pay for that day: having a contract stating that they're a) employed by the company on that date and b) paid on bank holidays. If the company didn't want that day covered in the contract, the time for them to do something about it was before they made the contract.

@DJClayworth They're exactly the same. In both cases, the company tries to impose a smaller payment after negotiating and signing a contract.

@marcelm And the difference is that one involves a small amount of money and is a mistake easy to make, and the other involves a very large amount of money and is very unlikely to be a mistake.

I'd agree to starting on 2nd April, but ask for an additional day off in lieu.

In the US we don't have these weird Holidays :-)

There are a lot of good answers, so I'll just give advice in the case where you decide to resist. This only works in the case where you weren't already unemployed or had your last day already decided. "I'm sorry, but I already adjusted my last day to match April 1st as we discussed." If you can, something 1 step further would be better: "I purposefully scheduled these dates so that I would not have a gap in insurance(/etc)". Not sure how applicable that is, and probably even less so since the last working day was a Friday, but showing that you'll be affected (besides just pay) should help.