The preposition commonly used before the word "Sundays" in English is "on." This is because "on" is used to indicate a specific day of the week, as in "on Sundays." This usage is consistent with the general rule that prepositions like "on" are used to denote a particular day or time, while other prepositions like "in" or "at" are used for broader time frames or locations.

Data from the Corpus of Contemporary American English (COCA) supports this usage. In the COCA, the phrase "on Sundays" appears significantly more frequently than any other preposition followed by "Sundays." For instance, "on Sundays" has a frequency of 2,610 occurrences, while "in Sundays" has only 5 occurrences, and "at Sundays" has only 2 occurrences. This overwhelming preference for "on" before "Sundays" underscores the grammatical convention in English.