Vanguard suggests that by waiting until April 15th to make the prior year's IRA contribution, investors are losing 15 months of compounding investment returns on their money. Instead, contribute as early as you can afford to do so, even right after New Year's of the year in which the contribution is for. So, for 2020 Vanguard suggests contributing now rather than waiting until the deadline next year.
Over 30 years, if you contributed their theoretical $5500 into your IRA in January versus April 15 of the following year, you might earn more than $15,500 at modest 4% returns. Of course we can't predict the stock market and we certainly can't time it.
However, Vanguard suggests that if you can afford to put your money to work earlier in the year, you'll end up with more money when you retire.
We agree, which is why we enthusiastically urge everyone to set aside money in the prior year in preparation of celebrating the new year with an IRA contribution (and a tax free Roth IRA contribution is even more exciting).