Depreciation for what? A business asset? A rental asset? Something else?
Generally, depreciation for an asset is determined in the first year that asset is placed in service, and that's it. For business assets, you have two possibilities in that first year.
First there's the SEC 179 deduction which allows you to deduct up to a maximum amount on an asset in the first year it's placed in service. Sometimes that maximum amount can be the full cost of the asset. But it depends on the asset. For example, with a SCH C business the total section 179 deduction and depreciation you can deduct for a passenger automobile, including a truck or van, you use in your business and first placed in service in 2020 is $18,100, if the special depreciation allowance applies, or $10,100, if the special depreciation allowance does not apply. Note that SEC179 can not be used for residential rental real estate assets. (but it can be used for some business rental real estate assets.) If an asset qualifies for SEC179, it must be used in the first year that asset is placed in service, and no other.
Then there's the Special Depreciation Allowance. This allowance is an additional deduction you can take after any SEC 179 deduction and before you figure regular depreciation under MACRS for the year you place the property/asset in service.
Both the SEC 179 deduction and the Special Depreciation Allowance must be used (if the property/asset qualifies) in the first year the property is placed in service.
More details on depreciation can be found in IRS Publication 946 - How To Depreciate Property at https://www.irs.gov/pub/irs-pdf/p946.pdf
