You can cash/deposit the check any time you want. But it does not change the fact that you were paid the money in 2019 and it will be included as a part of your 2019 income. Basically, you will have direct control of that money the day you get the check. Therefore, no matter when you cash it, that money is included in your 2019 income. The only thing you can do to change that, is to tell the employer to keep the severance pay because you don't want it.
