Bottom line: pick the Christmas season (mid-November to early January) for dreamy lights and Christmas markets; pick spring (March-May) for comfortable weather and flexibility. 1) Christmas season kicks off in mid-November when the Champs-Elysees lights switch on and runs to early January - hundreds of trees draped in lights, with Christmas markets around the Tuileries and Notre-Dame area - pure festive magic, but cold, short days and bigger crowds. 2) Spring (especially April-May) brings warmer weather and blooms, with longer days perfect for walking and sidewalk cafes - the most reliably pleasant season. 3) For thin crowds plus decent weather, early March and late October are the value windows. 4) If you go only once and want the safest bet, most guides recommend spring; if you want the most iconic romantic Paris glow, nothing beats Christmas.