Yes, the ACA marketplace subsidies are based on your entire year's income.
However, Medicaid looks at your month-to-month income. If you live in a state that adopted the ACA's Medicaid expansion and can get your monthly income down below 138% of the poverty level for that half year you should become eligible for that.
Conversely, if you prefer not to be pushed onto Medicaid, you may want to spread your Roth conversions/capital gains/etc. throughout the year rather than doing one lump sum.
