Isofield specific heat, $C(T,H)$, of single-crystal ${\mathrm{Ho}}_{1\ensuremath{-}x}{\mathrm{Y}}_{x}{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ ($x=0$, 0.25, 0.5, 1) were measured within the ranges $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}<T<50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $H(\ensuremath{\parallel}a)\ensuremath{\leqslant}60\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$. Linearized spin-wave theory is invoked to analyze the field and temperature dependence of the magnetic specific heat and entropy of ${\mathrm{HoNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ for $T<{T}_{N},H<{H}_{1}$ and $T<{T}_{N}$, $H>{H}_{3}$. Based on the known low-temperature $H\text{\ensuremath{-}}T$ phase diagram of ${\mathrm{HoNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$, this analysis identifies three distinct field regions. (i) The low-field region $(H<{H}_{1})$ where the collinear, commensurate $\ensuremath{\nearrow}\ensuremath{\swarrow}$ structure is stabilized, and ${C}_{M}(T<{T}_{N},H)$ and ${S}_{M}(T<{T}_{N},H)$ are well described by the prediction of linearized antiferromagnetic spin-wave analysis. In particular, ${(\ensuremath{\partial}{S}_{M}∕\ensuremath{\partial}H)}_{T}>0$, indicating that cooling can be effected by adiabatic magnetization. (ii) The intermediate-field region $({H}_{1}<H<{H}_{3})$, where the two metamagnetic states (namely $\ensuremath{\nearrow}\ensuremath{\nearrow}\ensuremath{\swarrow}$ at ${H}_{1}$ and $\ensuremath{\nearrow}\ensuremath{\nearrow}\ensuremath{\nwarrow}$ at ${H}_{2}$) are stabilized. Here no spin-wave analysis is attempted, but it is evident that ${(\ensuremath{\partial}{S}_{M}∕\ensuremath{\partial}H)}_{T}>0$ for $H<{H}_{2}$ while ${(\ensuremath{\partial}{S}_{M}∕\ensuremath{\partial}H)}_{T}<0$ for $H>{H}_{2}$. (iii) The high-field region $(H>{H}_{3})$ where the saturated $\ensuremath{\nearrow}\ensuremath{\nearrow}$ state is being approached. Here, both ${C}_{M}(T<{T}_{N},H)$ and ${S}_{M}(T<{T}_{N},H)$ follow the description of a ferromagnetic spin-wave analysis; furthermore, ${(\ensuremath{\partial}S∕\ensuremath{\partial}H)}_{T}<0$, indicating that cooling can be effected by adiabatic demagnetization. The magnetocaloric effect above ${T}_{N}$ as well as alloying influences on the magnetic properties are discussed.