The paper examines the results of phenological research on common beech (Fagus sylvatica L.) during a period of 21 years (1995–2015) in the submontane beech forest of central Slovakia (Inner Western Carpathians). We focused on bud-burst, leaf unfolding and leaf colouring. Temporal analysis indicated that the mean monthly air temperature increased, especially from April to August. An extraordinary increase of air temperature in March and April, mostly in the last decade, was detected. The precipitation from May to August varied considerably, but in the range of the long-term mean value. During the study period, the mean/earliest/ latest onset of the bud-burst of common beech was observed on the 110^th /101^st/120^th day of the year (DOY), respectively. As for leaf unfolding 10% and 50% (LU 10 and LU 50), we found the mean/earliest/latest onset on the 114^th/103^rd/122^nd DOY and on the 118^th/108^th/124^th DOY, respectively. The mean/earliest/latest onset of leaf colouring 10% (LC 10) and 50% (LC 50) started on the 272^nd/262^nd/288^th DOY and on 286^th/276^th/298^th, respectively. A medium degree of negative correlation (r = -0.68, P < 0.05) was found between air temperature and spring plant development (LU 50). In contrast, for both the cumulative temperature and precipitation, we found very low correlation with autumnal leaf phenology (r ≤ 0.3, P > 0.05). The vegetation period of the examined tree species lasted for 168 days on average (min/max were 155/183 days). Trend analysis revealed an earlier onset of spring phenophases by 7 days/2 decades. Conversely, a delay of autumnal phenophases by 9 days was recorded, so the vegetation period of beech extended by more than two weeks during the study period.