Times for 40-Day Segulot

Hello.
When one undertakes to do/read something for 40 days as a segulah (spiritual remedy), what are the times during the day considered as a specific day and from when does that day pass and it no longer counts (if it happened)?
The question arose from the segulah to light a candle for our holy matriarch Rachel, as when we reach the weekend, we light on Friday before Shabbat and then after Havdalah on Motzei Shabbat (Saturday night), since Shabbat evening is already Sunday, it turns out we didn't actually light on Shabbat. How does this align and does it apply to other 40-day segulot?
Thank you very much!