Question: Why do women cover their eyes when they light Shabbat candles?
Answer: This custom is a means of overcoming a halachic dilemma. Normally blessings are supposed to precede the performance of a mitzvah. However, if a woman recites “l’hadlik ner shel Shabbat” before she lights, she will be assuming Shabbat upon herself and not be allowed to light Shabbat candles anymore. Reciting the blessing afterwards, on the other hand, would violate the rule that blessings should precede the performance of the mitzvah.
To get around this problem, women light Shabbat candles first and then recite the blessing while covering their eyes. When they open them, they are enjoying the lights of the candles for the “first” time. Thus, in a sense, the blessing precedes the act as per the general rule.
If this is the reason for covering one’s eyes, however, why do women cover their eyes when lighting Yom Tov candles? Lighting candles is permitted on Yom Tov, and women could recite the blessing before lighting.
Indeed, some women do change their practice and recite the blessing first when lighting Yom Tov candles. Others don’t however. The rationale for this custom is that changing one’s practice for Yom Tov would be confusing.