Either or Both

[Nami Nami]

If someone asked me which day I am available, normally we use either or both to indicate I am actually availble on both days?

e.g. Hi Nami Nami, are you availble on Friday or Saturday?

1. I am available on either day.
2. I am available on both days.

Which reply is more common or natural?

[Alan]

In this context they are identical, although naturally that would not always be so.

Both are possible & natural.