Timing of birth is one of the key mechanisms by which female mammals with seasonal reproduction can maximize their fitness. The aim of our study was to test the effect of maternal condition on the optimization of birth date in a model monotocous, seasonally polyestrous mammal – the red deer (Cervus elaphus). During 20 years (1998-2017; experimental, deer farm VÚŽV Podlesko), we, therefore, studied the effects of selected factors on (1) relative delivery date of the mother (number of days from the 1st May till delivery), (2) the index of delivery synchrony of the mother (the average number of days from the delivery till deliveries of other females), the body weight of the offspring (3) at birth and (4) at weaning. We assumed that (a) mothers in good condition will deliver earlier and will be more synchronous than mothers in poor condition, (b) male offspring will born earlier than female offspring, (c) earlier borne offspring will be heavier both at birth and at weaning. Using General Linear Mixed Models (GLMMs) we analyzed records from 98 mother and 385 offspring. As we expected, the relative delivery date of the mother increased with the decreasing body weight of the mother before conception (p < 0.01) and with decreasing dominance rank of the mother at delivery (p < 0.01). The index of delivery synchrony also increased with the decreasing body weight of the mother before conception (p = 0.02) and with decreasing dominance rank of the mother at delivery (p < 0.01). Despite that male offspring weighed more at birth than female offspring (p < 0.01), the sex of the offspring did not affect its birth date (NS). The body weight of the offspring at birth was not affected by the birth date (NS), this effect was, however, detected at weaning (p = 0.02). The earlier the offspring was born the more it weighed at weaning. Owing to earlier deliveries, the mothers in good condition provided their offspring optimal conditions for their development and invested in them longer than the mothers in poor conditions. Due to this, they increased not only the condition and prospect of survival of their offspring but also their reproductive potential.