When a preposition ‘в’ turns into ‘во’

Why do we say ‘в понедельник’, ‘в среду’, ‘в четверг’ and then ‘во вторник’? Why is it ‘во Франции’ and ‘во время’? This is the case when there is an easy answer to those questions!

The preposition ‘в’ turns into ‘во’ when the following word begins with two consonants, the first of which is either В or Ф. It’s quite challenging to pronounce those three consonants together. An extended preposition ‘во’ makes the task easier.

Go through the examples and see for yourself.

Во вторник // В воскресенье
Во Владивостоке // В Вашингтоне
Во Франции // В Финляндии
Во вчерашней газете // В вечерних новостях

For the same phonetic reason we use the preposition ‘во’ with some monosyllabic words beginning with two consonants (во сне, во дворе, во рту, во льду) and with words beginning with МН (во мне, во многом, во многих городах).