'On Monday', 'In Monday', or Just 'Monday'?

It's "on Monday." For days and dates, we use the preposition "on". In casual situations, many natives omit the preposition before the day of the week.

'On Sunday', 'In Sunday', or Just 'Sunday'?

It's on Sunday. For days and dates, we use the preposition "on". In casual situations, many natives omit the preposition before the days of the week.

'On Friday' or 'On Fridays'?

Use the plural form Fridays to talk about every Friday or "Fridays" in general.

'On Monday' or 'On Mondays'?

Use the plural form Mondays to talk about every Monday or "Mondays" in general.

'On Sunday' or 'On Sundays'?

Use the plural form "Sundays" to talk about every Sunday or "Sundays" in general.