This study was conducted in central and northern Ivory Coast, precisely in Yamoussoukro and Ferkessédougou with the onion variety Ares. The objective was to determine the best planting dates for rainfed onion production in each of the agro-ecological zones. Thus, five sowing dates were tested. In Yamoussoukro, sowing was carried out on 01 May (D1), 15 May (D2), 01 June (D3), 15 June (D4) and 01 July. In Ferkessédougou, sowing was carried out on 3 May (D1), 17 May (D2), 3 June (D3), 17 June (D4) and 3 July (D5). The parameters evaluated were the survival rate of the plants after transplanting, the height of the plants, the number of leaves emitted from the plants, the rate of bulb rot at harvest, the total bulb yield and the weight per month of the bulbs. At the end of the trials, the best plant survival rates after transplanting were obtained for sowing dates D3 (94.38%), D4 (92.86%) and D2 (88.23%) in Yamoussoukro and sowing dates D1 (92.28%), D2 (91.1%) and D3 (89.15%) in Ferkessédougou, respectively. Plant height, number of leaves produced and average bulb weight decreased from early sowing to late sowing. Bulb rot rates were higher for early sowings and zero for late sowings. In Yamoussoukro, the best yields were obtained for sowing dates D3 (23.56 t/ha), D4 (22.42 t/ha) and D2 (20.27 t/ha). As for Ferkessedougou, the best yields were obtained for sowing dates D2 (23.72 t/ha) and D3 (23.15 t/ha).