Although boreal peatlands play a critical role in the global carbon cycle, their year-round carbon dioxide (CO₂) dynamics — and particularly the contribution of the non-growing season (NGS) — remain poorly constrained in annual balance estimates. Using 17 years (2005–2021) of eddy covariance measurements from a fen in southern Finland, we first quantified the magnitude, timing, and interannual variability of CO₂ fluxes. We then examined in greater detail the NGS, with particular emphasis on soil temperature dynamics and the role of thermal legacy effects. On average, the NGS accounted for 60 % of the year (226 ± 27 days), ranging from mid-September to late April, and offset 57 % of the subsequent growing season’s (GS) CO₂ uptake. NGS emissions declined from autumn to spring, with the highest carbon emissions occurring across September–December and the lowest in January–February. Soil temperature—both concurrent and lagged up to four months—was the main control of CO₂ fluxes during November–December and spring thaw, while photosynthetically active radiation (PAR) dominated during the onset of the NGS. Variability in annual CO₂ balances was large, and in two years (2016 and 2018) the fen switched from a net CO₂ sink to a source. Finally, we focused on 2016 in detail: an exceptional six-week CO₂ release during April–May released 84 g C m⁻², offsetting 38 % of the following GS CO₂ uptake. This event was linked to unusually warm late-autumn soils, minimal snow insulation, and subsequent rapid surface freezing, which likely enhanced CO₂ accumulation and stimulated CO₂ release during thaw. Our results demonstrate that short-lived but intense NGS events can determine the annual peatland CO₂ balance and therefore significantly affect the annual carbon budget of boreal peatlands.