The right time for #passover is always after the first ripe barley, or else the #firstfruits commands don't make sense. It doesn't necessarily have to be immediately after the first ripe barley--there's no reason it can't be a full month or more later--but it can't be earlier, unless there's a mini ice age or something going on. The Hebrew calendar adds a month approximately every three years to keep it in sync with the solar year and barley harvest.