The answer to the question"When does IRS start accepting returns" depends upon several situations. I shall discuss every situation differently as follows:

When you receive your W2- when you receive your W2 from all your employers, you can prepare & submit your tax return anytime before April 15 without filing an extension. But if your are not able to file a tax return by April 15, then to avoid interest & penalty etc, you should file an extension and never forget to pay the amount of tax due, if any with your extension. This shall give you an extension of 6 months & if you have paid the amount of tax with your extension that is higer than the amount of tax due with your tax return, you won't be hit by interest & penalty, infarct you will get the extra amount paid back by IRS with interest.

When you receive Form 1099- You must file a tax return on receiving a Form 1099 & you can file it any time after receiving it & after your receive proof of all other incomes like W2. It is very important to file a tax return after receiving a Form 1099 because one copy of 1099 is submitted to the IRS by your payer. So IRS already has your proof of income. There can be various types of Form 1099:

    Form 1099-B
    Form 1099-C
    Form 1099-DIV
    Form 1099-G
    Form 1099-INT
    Form 1099-MISC
    Form 1099-MSA
    Form 1099-OID
    Form 1099-PATR
    Form 1099-R
    Form 1099-S
    Form SSA-1099
    Form RRB-1099

So, with this above listed information in your hand, go ahead & file your tax return on time before April 15 or after April 15 with an extension.