Timing for planting onions on turnips

You can grow onions from seeds, from which small onions - sets - are obtained in the first year. The sets are planted in the spring and the harvest of ordinary onion bulbs is harvested in the fall. The onion harvest directly depends on the timing of its planting. But what is the optimal time for planting onions on turnips to get a good harvest?
Onion sets are planted in early May, but while the soil is not warmed up (below +12 °C), it is better to wait. It is necessary to prepare the soil for planting onions in advance. If you plant onions on turnips earlier, the bulbs will germinate at low temperatures, but most plants will germinate. Early planting of onions creates conditions for the development of a powerful root system. It is not advisable to be late in planting onions, since due to lack of moisture and high temperature, the top layer of soil dries out very quickly, and the bulbs take root poorly, which reduces the yield of the bulbs. In the middle zone, the best time for planting onions is May 10. First of all, it is better to plant small onion sets, then there will be a high probability that the plants will not shoot arrows. Later, medium onions are planted, and then large ones. The indicator for planting onions is the flowering of bird cherry.
Comments
I didn’t know that small onions for sowing are called sets, and I’ve never come across the expression planting onions on a turnip before. For many years now I have been planting onion sets in the first week of April, and almost every year I have a normal harvest.