をもって / をもちまして

をもって follows a noun and has two jobs. After a date or an event it marks the point where something ends: "as of", "with this". After a noun like せいい誠意 or しょめん書面 it marks the means: "with", "by". Both belong to notices, announcements, ceremonies and business letters, and をもちまして is the spoken polite form used to an audience.

The end point

This is the use you will see most, on shop doors and in farewell emails. The noun is a date, a period or an event, and the verb says something stops or takes effect: closing, retiring, ending a service.

In everyday speech the same thing is で: きょう今日でみせ店をしめます閉めます.

The means

With an abstract noun – せいい誠意 "sincerity", ぜんりょく全力 "all one's strength", しょめん書面 "writing", たすうけつ多数決 "a majority vote" – をもって says how something is done. It is a formal で.

Usage notes

These go beyond an end point and a means.

As the grounds for a decision

In official writing をもって can name what a decision rests on.

Spelling

The もって here is the old word もって以て, and very formal text still writes it that way. Read in context, it is not the verb もつ持つ "to hold".